Magneto-optical recording medium having large kerr rotational angle in short wavelength range

ABSTRACT

In a magneto-optical recording medium, a protective layer 14, a first magnetic layer 11 formed of a light rare earth element--heavy rare earth element--transition metal alloy, a second magnetic layer 12 formed of a light rare earth element--heavy rare earth element--transition metal alloy, a third magnetic layer 13 formed of a light rare earth element--heavy rare earth element--transition metal alloy, another protective layer 15, and a reflection layer 16 are laminated in sequence on a transparent substrate 10. The first, second and third magnetic layers are sandwiched so as to form a recording film 17. The compositions of the first and third magnetic layers are so selected as to provide a large Kerr rotational angle in a short wavelength range (400 to 700 nm), which is high in the ratio of light rare earth element. On the other hand, the composition of the second magnetic layer is so selected as to be prominent in the characteristics for forming a perpendicular magnetized film, which is high in the ratio of heavy rare earth element. The recording film 17 formed by sandwiching the first, second and third magnetic layers has a large Kerr rotational angle in a short wavelength range as a whole, and further provides a high coercivity. The recording film 17 is formed thin (400 angstrom or less) so that light can transmit sufficiently therethrough. Further, when the three layers of sandwich structure is combined with the reflection layer 16, a more larger Kerr rotational angle can be obtained.

This is a divisional of application Ser. No. 08/090,040 filed on Jul.19, 1993, now U.S. Pat. No. 5,648,161.

TECHNICAL FIELD

The present invention relates generally to a magneto-optical recordingmedium to and from which information signals are written and read byutilization of magneto-optical (Kerr) effect, and more specifically tothe magneto-optical recording medium suitable for a high densityrecording with the use of a laser beam of short wavelength.

BACKGROUND ART

At present, recording media based upon the magneto-optical recordingtechnique have widely been put into practical use. In themagneto-optical recording method, the magneto-optical effect of a thinfilm formed of a heavy rare earth element--transition metal alloy ofperpendicular magnetic anisotropy is utilized for realization ofpractical rewritable recording media of a large capacity. The recordingmedium of this type is of portability, so that there exists such meritthat recorded information can be kept secret or recorded information oflarge capacity can be moved easily.

With the advance of the development of multimedia for recording imagedata for instance, further increased memory capacitance is required moreand more for the magneto-optical recording media. For satisfying thisrequirement, at present the higher density technique is now beingresearched on the basis of a laser beam of shorter wavelength (400 to700 nm). One of the technical problems related to this technique is toobtain magnetic material which has a large Kerr rotational angle in asshort a wavelength range as 400 to 700 nm. One of these materials isamorphous alloy which contains a light rare earth element (e.g., NdFeCo)and a transition metal. In the amorphous alloy, since the 4f electronscontribute to an increase in the magneto-optical effect, the Kerrrotational angle is large in the short wavelength range (J. Mag. Soc.Jpn. 11 Suppl. S1. 273 (1991)), so that this alloy is an effectivecandidate for the high density recording media.

In the NdFeCo, since the magnetization is large, there exists a problemin that it is difficult to form the perpendicular magnetized filmrequired for the magneto-optical medium. To overcome this problem,recently there has been proposed such a method that a NdCo layer whichhas large Kerr rotational angle in a short wavelength range (not formedinto a perpendicular magnetized film) is sandwiched between two TbFeColayers (formed into perpendicular magnetized films) for providing mutualexchange coupling so that the three magnetic layers can function as aperpendicular magnetized film (Journal of Applied Physics Vol. 69 p4761,by Iiyori et al. of IBM Japan). Hereinafter, the structure such that anon-perpendicular magnetized layer is sandwiched between the twoperpendicular magnetized layers as described above is referred to assandwich structure.

In the prior art sandwich structure such that the NdCo layer issandwiched between the two TbFeCo layers, however, since laser beamsignals must be detected from the NdCo layer through the TbFeCo layerwith an about 100 angstrom thickness, there exists a problem in that asufficiently large Kerr rotational angle is not obtained in the case ofa laser beam in the short wavelength range (400 to 700 nm).

On the other hand, in order to provide a large Kerr rotational angle inresponse to a laser beam, a reflection structure is so far well known.In this structure, a light reflection layer formed of material such asAl is formed in the recording medium, and a magnetic layer for producinga large Kerr rotational angle is formed on the surface of the lightreflection layer. A laser beam outputted from an optical head is passedthrough the magnetic layer, reflected from the reflection layer, andagain passed through the magnetic layer, before received by the opticalhead. In this method, since the laser beam is passed through themagnetic layers twice, it is possible to provide a large Kerr rotationalangle to the laser beam. In the prior art sandwich structure as alreadydescribed, however, since the magnetization of the middlenon-perpendicular magnetized film must be stood in the verticaldirection by the strong exchange coupling force applied from theperpendicular magnetized films provided on both the sides, the thicknessof at least one side perpendicular magnetized film must be as thick asabout 1000 angstrom.

As a result, in the prior art sandwich structure, it is impossible totransmit a laser beam therethrough, and thereby it is impossible tocombine the sandwich structure with the afore-mentioned reflectionstructure.

DISCLOSURE OF THE INVENTION

The first object of the present invention is to provide amagneto-optical recording medium by which a sufficiently large Kerrrotational angle can be obtained in a short laser wavelength range (400to 700 nm).

The second object of the present invention is to enable the sandwichstructure and the reflection structure to be combined with each other,in order to obtain a sufficiently large Kerr rotational angle in a shortlaser wavelength range (400 to 700 nm), by reducing the thicknesses ofthe respective layers on the basis of novel composition of the threemagnetic layers of sandwich structure.

The third object of the present invention is to enable the two exchangecoupled magnetic layers and the reflection structure to be combined witheach other, in order to obtain a sufficiently large Kerr rotationalangle in a short laser wavelength range (400 to 700 nm), by exchangecoupling a thin single-layer perpendicular magnetized film with anon-perpendicular magnetized film of large Kerr rotational angle so asto stand the magnetization of the non-perpendicular magnetized film inthe vertical direction.

According to the present invention, as the magnetic film material of themagneto-optical recording medium, light rare earth element--heavy rareearth element--transition metal alloys are used. Since the magnetic filmformed of a light rare earth element--heavy rare earthelement--transition metal alloy includes a light rare earth element(e.g., Nd or Pr or Sm), the Kerr rotational angle in the shortwavelength range is relatively large, as compared with the magnetic filmformed of an alloy such as TbFeCo including a heavy rare earth element(e.g., Dy or Tb or Gd) but not including the light rare earth element,as shown in FIG. 11 (in which the magnetic film is formed on a glasssubstrate and the Kerr rotational angle is measured on the substrateside). Further, in the magnetic film formed of the light rare earthelement--heavy rare earth element--transition metal alloy, when thecomposition is selected appropriately, it is possible to obtain themagnetic film provided with characteristics as the perpendicularmagnetized film, coercivity of about 10 kOe at room temperature, andCurie temperature between 150° and 200° C., with the result that themagnetic film is suitable also for use as the magneto-optical recordingmedium of single layer structure. In other words, the light rare earthelement--heavy rare earth element--transition metal alloy of appropriatecomposition is excellent in the signal recording characteristics inthermo-magnetic writing and further the stability in recording domains.

In the present specification, the composition of the light rare earthelement--heavy rare earth element--transition metal alloy is classifiedinto a range (referred to as heavy rare earth major range, hereinafter)in which the content ratio of the heavy rare earth element is relativelylarge, and another range (referred to as light rare earth major range,hereinafter) in which the content ratio of the light rare earth elementis relatively large. An example of the composition included within theheavy rare earth major range is Nd6Dy23Fe57Co14 (at %). In thecomposition as described above, the characteristics as the perpendicularmagnetized film is prominent and a relatively large coercivity can beobtained. In this composition, if the Nd and Co are increased and Dy andFe are decreased, the composition enters the light rare earth majorrange. An example thereof is Nd25 Dy8 Fe35 Co32 (at %). In thiscomposition, although the coercivity is as small as 0.8 kOe, a largeKerr rotational angle can be obtained in the short laser wavelengthrange. FIG. 12 shows the dependence of the wavelength upon the Kerrrotational angle in these two composition examples (the magnetic filmsare formed on a glass substrate and the Kerr rotational angles aremeasured from the substrate side).

According to the present invention, the magnetic film of themagneto-optical recording medium is a film (referred to as a multilayerfilm, hereinafter) formed by laminating a plurality of magnetic layershaving respective compositions of both the heavy rare earth major rangeand the light rare earth major range. By adopting the above-mentionedmultilayer film, it is possible to obtain the recording medium whosecoercivity is as large as being equivalent to the composition of theheavy rare earth major range and whose Kerr rotational angle in theshort laser wavelength range is as large as being equivalent to thecomposition of the light rare earth major range.

The multilayer film of the present invention can be adopted as thereproducing film for the magnetic induced super resolution reproducingmedium and the direct overwrite medium utilizing the exchange-coupledtwo layer films, so that it is possible to provide recording medium ofhigh performance.

Further, the recording medium of the present invention adopts thereflection structure formed by laminating the reflection layer on theabove-mentioned multilayer film. Accordingly, it is possible to furtherenhance the Kerr rotational angle, thus providing a magneto-opticalrecording medium of higher performance index.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view showing an embodiment 1 of thepresent invention;

FIG. 2 is a side cross-sectional view showing a comparative example usedfor the evaluation test of the embodiment 1;

FIG. 3 is a side cross-sectional view showing an embodiment 2 of thepresent invention;

FIG. 4 is a side cross-sectional view showing an embodiment 3;

FIG. 5 is a side cross-sectional view showing a comparative example usedfor the evaluation test of the embodiments 2 and 3;

FIG. 6 is a side cross-sectional view showing an embodiment 4;

FIG. 7 is a side cross-sectional view showing a comparative example usedfor the evaluation test of the embodiment 4;

FIG. 8 is an illustration for assistance in explaining the magnetizationdirections of the recording domains formed in the magnetic layers;

FIG. 9 is a side cross-sectional view showing an embodiment 5;

FIG. 10 is a side cross-sectional view showing a comparative exampleused for the evaluation test of the embodiment 5;

FIG. 11 is a graphical representation showing the err rotational anglesof NdDyTbFeCo and TbFeCo;

FIG. 12 is a graphical representation showing the Kerr rotational anglesof Nd base alloy in both the heavy rare earth major range and the lightrare earth major range;

FIG. 13 is a side cross-sectional view showing an embodiment 6;

FIG. 14 is a graphical representation showing the Kerr rotational anglesof the embodiment 6 and a comparative example;

FIG. 15 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 6 and the comparativeexample;

FIG. 16 is a graphical representation showing the Kerr rotational anglesof the embodiment 7 and a comparative example;

FIG. 17 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 7 and the comparativeexample;

FIG. 18 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 7 and the comparativeexample, which are obtained when Co content is the same in both themagnetic layers 61 and 63;

FIG. 19 is a side cross-sectional view showing an embodiment 8;

FIG. 20 is a side cross-sectional view showing the comparative exampleused for the evaluation test of the embodiment 8;

FIG. 21 is a graphical representation showing the Kerr rotational anglesof the embodiment 8 and a comparative example;

FIG. 22 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 8 and the comparativeexample;

FIG. 23 is a side cross-sectional view showing an embodiment 9;

FIG. 24 is a side cross-sectional view showing the comparative exampleused for the evaluation test of the embodiment 9;

FIG. 25 is a graphical representation showing the Kerr rotational anglein the embodiment 9 and the comparative example;

FIG. 26 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 9 and the comparativeexample;

FIG. 27 is a side cross-sectional view showing an embodiment 10;

FIG. 28 is a graphical representation showing the Kerr rotational anglesof the embodiment 10 and a comparative example;

FIG. 29 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 10 and the comparativeexample;

FIG. 30 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 11 and the comparativeexample;

FIG. 31 is a side cross-sectional view showing an embodiment 12;

FIG. 32 is a perspective view showing the structure of an instrument forforming artificial lattice;

FIG. 33 is a graphical representation showing the Kerr rotational anglesof the embodiment 12 and a comparative example;

FIG. 34 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 12 and the comparativeexample;

FIG. 35 is a side cross-sectional view showing an embodiment 13;

FIG. 36 is a graphical representation showing the Kerr rotational anglein the embodiment 13 and a comparative example;

FIG. 37 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 13 and the comparativeexample;

FIG. 38 is a side cross-sectional view showing an embodiment 14;

FIG. 39 is a graphical representation showing the Kerr rotational anglesof the embodiment 14 and a comparative example;

FIG. 40 is a graphical representation showing dynamic characteristics inthe short wavelength range of both the embodiment 14 and the comparativeexample; and

FIG. 41 is a side cross-sectional view showing an embodiment 15.

BEST MODES FOR EMBODYING THE INVENTION

The preferred embodiments of the present invention will be describedhereinbelow. In the embodiments described below, the coercivity of thesingle magnetic layer indicates the value obtained by measuring themagnetic layer formed by laminating a single magnetic layer with athickness of 200 angstrom and a AlSiN protective layer with a thicknessof 800 angstrom on a polycarbonate substrate in sequence. Further, theKerr rotational angle of the single magnetic layer indicates the valueobtained by measuring, from the substrate side, the magnetic layerformed by laminating a single magnetic layer with a thickness of 1000angstrom and a AlSiN protective layer with a thickness of 800 angstromon a polycarbonate substrate in sequence. Further, the S/N ratioindicates the narrow band S/N ratio measured within the band width 30kHz.

EMBODIMENT 1 (3-LAYER SANDWICH+REFLECTION STRUCTURE)

FIG. 1 is a side cross-sectional view showing the embodiment 1. A firstprotective layer 14 of 600 angstrom in thickness, a first magnetic layer11 of 60 angstrom in thickness, a second magnetic layer 12 of 80angstrom in thickness, a third magnetic layer 13 of 60 angstrom inthickness, a second protective layer 15 of 200 angstrom in thickness,and a reflection layer 16 of 600 angstrom in thickness are laminated ona polycarbonate substrate 10 in sequence by magnetron sputtering. Themagnetic layers 11, 12 and 13 are amorphous.

The examples of the material of the respective layers are as follows:the material of the first and second protective layers 14 and 15 isAlSiN; that of the reflection layer 16 is Al; that of the first, secondand third magnetic layers is a light rare earth element heavy rare earthelement--transition metal alloy such as NdDyFeCo, NdTbFeCo, NdTbDyFeCo,PrDyFeCo, PrTbFeCo or PrTbDyFeCo. Here, the composition of the secondmagnetic layer 12 is selected from the heavy rare earth major range. Onthe other hand, the composition of the first and third magnetic layers11 and 13 are the same composition selected from the light rare earthmajor range. The examples of composition included in the heavy rareearth major range are listed in Table 6, and the examples of compositionincluded in the light rare earth major range are listed in Tables 1 to5. The above-mentioned first, second and third magnetic layers 11, 12and 13 are exchange-coupled with respect to each other and constitutes amagnetic film 17 (referred to as a recording film, hereinafter) forrecording information as a whole.

When the above-mentioned compositions are adopted for the magneticlayers 11, 12 and 13, the first and third magnetic layers 11 and 13 havepreferable characteristics such that the Kerr rotational angle issufficiently large in the short wavelength range. Further, the first andthird magnetic layers 11 and 13 are weak (i.e., the coercivity is small)in the characteristics as the perpendicular magnetized film, as comparedwith the second magnetic layer 12. However, the first and third magneticlayers 11 and 13 are strong in the characteristics as the perpendicularmagnetized film, as compared with the light rare earthelement--transition metal alloy (e.g., NdCo) (including no heavy rareearth element) used as the magnetic layer having a large Kerr rotationalangle in the prior art sandwich structure. On the other hand, the secondmagnetic layer 12 is provided with a square-shaped hysteresis loop, andtherefore the coercivity is large. In other words, the second magneticlayer 12 is strong in the characteristics as the perpendicularmagnetized film. In addition, the second magnetic layer 12 is small inthe Kerr rotational angle in the short wavelength range, as comparedwith the first and third magnetic layers 11 and 13. However, the secondmagnetic layer 12 is large in the Kerr rotational angle, as comparedwith the heavy rare earth element--transition metal alloy (e.g., TbFeCo)(including no light rare earth element) used as the magnetic layerhaving a large coercivity in the prior art sandwich structure.

Owing to the characteristics of the respective magnetic layers asdescribed above, it is possible to adopt an advantageous structuredifferent from the prior art sandwich structure. In more detail,firstly, in the prior art sandwich structure, a magnetic layer of alarge Kerr rotational angle is sandwiched between the two magneticlayers of a large coercivity. In the present invention, as shown in theembodiment 1, it is possible to adopt such a sandwich structure that amagnetic layer 12 of a large coercivity is sandwiched between the twomagnetic layers 11 and 13 of a large Kerr rotational angle. As a result,it is possible to obtain a larger Kerr rotational angle in the shortwavelength range, as compared with the prior art sandwich structure.Secondly, since the magnetic layers 11 and 13 having a large Kerrrotational angle, respectively have a large coercivity, as compared withthose of the prior art sandwich structure, it is possible to extremelyreduce the thickness of the magnetic layer 12 having a large coercivity,as compared with that of the prior art sandwich structure. Accordingly,the total film thickness of the recording film 17 can be reducedsufficiently thin so that a laser beam can be transmitted therethrough,thus allowing the adoption of reflection structure and thereby the Kerrrotational angle can be further increased.

The evaluation test results of the embodiment 1 shown in FIG. 1 will bedescribed hereinbelow.

FIG. 2 is a side cross-sectional view showing a comparative example usedfor the evaluation test. A first protective layer 14 of 600 angstrom inthickness, a recording film 12' of single layer structure with athickness of 200 angstrom, a second protective layer 15 of 200 angstromin thickness, and a reflection layer 16 of 600 angstrom in thickness arelaminated on a polycarbonate substrate 10 in sequence by magnetronsputtering. The material of the first and second protective layers 14and 15 is AlSiN, and that of the reflection layer 16 is Al. Therecording film 12' is of an alloy Nd6Dy22Fe52Co20 (at %) of heavy rareearth major range composition. In the single layer of the recording film12', the Kerr rotational angle is 0.250 and the coercivity is 10.5 kOein laser wavelength of 500 nm.

With respect to the embodiment 1 shown in FIG. 1, the composition of thesecond magnetic layer 12 is the same as that (Nd6Dy22Fe52Co20 (at %)) ofthe recording film 12' of the comparative example. Further, with respectto the first and second magnetic layers 11 and 13, 40 samples ofdifferent composition as listed in Tables 1, 2, 3, 4 and 5 wereprepared. These samples are referred to as the example 1-1 to example1-40 hereinafter. Further, in Tables 1 to 5, the Kerr rotational anglesθs in the laser wavelength of 500 nm and the coercivities Hcs of thefirst and third magnetic layers of single layer structure are bothlisted for each composition of 40 examples.

                  TABLE 1                                                         ______________________________________                                        SAMP No. 1st & 3rd MAG                                                        LAYER COMP     θs                                                                              Hcs      θm                                                                            Hcm                                     (at %)         (degree)                                                                              (kOe)    (degree)                                                                            (kOe)                                   ______________________________________                                        EXAMPLE 1-1    0.37    0.3      1.14  4.0                                     Nd15 Tb5 Fe45 Co35                                                            EXAMPLE 1-2    0.33    0.4      1.09  5.2                                     Nd15 Tb8 Fe42 Co35                                                            EXAMPLE 1-3    0.32    0.6      1.02  5.8                                     Nd15 Tb10 Fe40 Co35                                                           EXAMPLE 1-4    0.39    0.4      1.19  5.0                                     Nd23 Tb5 Fe34 Co38                                                            EXAMPLE 1-5    0.36    0.6      1.23  6.2                                     Nd23 Tb8 Fe31 Co38                                                            EXAMPLE 1-6    0.35    0.8      1.09  6.5                                     Nd23 Tb10 Fe29 Co38                                                           EXAMPLE 1-7    0.38    0.3      1.15  3.9                                     Nd30 Tb5 Fe25 Co40                                                            EXAMPLE 1-8    0.33    0.4      1.15  4.8                                     Nd30 Tb8 Fe22 Co40                                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        SAMP No. 1st & 3rd MAG                                                        LAYER COMP     θs                                                                              Hcs      θm                                                                            Hcm                                     (at %)         (degree)                                                                              (kOe)    (degree)                                                                            (kOe)                                   ______________________________________                                        EXAMPLE 1-9    0.32    0.6      0.97  5.5                                     Nd30 Tb10 Fe20 Co40                                                           EXAMPLE 1-10   0.35    0.6      1.14  5.9                                     Nd23 Dy8 Fe29 Co40                                                            EXAMPLE 1-11   0.35    0.7      1.21  6.2                                     Nd23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 1-12   0.36    0.3      1.05  4.3                                     Pr15 Tb5 Fe40 Co40                                                            EXAMPLE 1-13   0.36    0.7      1.09  5.7                                     Pr23 Tb8 Fe29 Co40                                                            EXAMPLE 1-14   0.32    0.4      1.07  4.9                                     Pr30 Tb8 Fe30 Co30                                                            EXAMPLE 1-15   0.35    0.6      1.15  5.5                                     Pr23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 1-16   0.35    0.4      1.13  5.7                                     Pr23 Dy8 Fe29 Co40                                                            ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        SAMP No. 1st & 3rd MAG                                                        LAYER COMP     θs                                                                              Hcs      θm                                                                            Hcm                                     (at %)         (degree)                                                                              (kOe)    (degree)                                                                            (kOe)                                   ______________________________________                                        EXAMPLE 1-17   0.26    0.2      1.02  3.0                                     Nd10 Tb3 Fe47 Co40                                                            EXAMPLE 1-18   0.37    0.2      1.22  3.5                                     Nd15 Tb3 Fe42 Co40                                                            EXAMPLE 1-19   0.39    0.2      1.20  4.0                                     Nd23 Tb5 Fe34 Co40                                                            EXAMPLE 1-20   0.39    0.2      1.22  2.9                                     Nd30 Tb3 Fe27 Co40                                                            EXAMPLE 1-21   0.38    0.2      0.88  2.3                                     Nd35 Tb3 Fe22 Co40                                                            EXAMPLE 1-22   0.28    0.4      1.75  4.8                                     Nd35 Tb10 Fe15 Co40                                                           EXAMPLE 1-23   0.26    0.8      0.72  5.5                                     Nd35 Tb15 Fe10 Co40                                                           EXAMPLE 1-24   0.28    1.0      0.73  7.2                                     Nd30 Tb15 Fe15 Co40                                                           ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        SAMP No. 1st & 3rd MAG                                                        LAYER COMP     θs                                                                              Hcs      θm                                                                            Hcm                                     (at %)         (degree)                                                                              (kOe)    (degree)                                                                            (kOe)                                   ______________________________________                                        EXAMPLE 1-25   0.29    1.2      0.85  7.9                                     Nd23 Tb15 Fe22 Co40                                                           EXAMPLE 1-26   0.27    1.0      0.83  6.9                                     Nd15 Tb15 Fe30 Co40                                                           EXAMPLE 1-27   0.26    0.8      0.78  5.8                                     Nd10 Tb15 Fe35 Co40                                                           EXAMPLE 1-28   0.29    0.4      0.88  4.1                                     Nd10 Tb10 Fe40 Co40                                                           *EXAMPLE 1-29  0.22    0.05     0.96  *0.8                                    Nd10 Tb1 Fe49 Co40                                                            *EXAMPLE 1-30  0.40    0.05     1.20  *1.0                                    Nd23 Tb1 Fe36 Co40                                                            *EXAMPLE 1-31  0.37    0.05     0.78  *0.7                                    Nd35 Tb1 Fe24 Co40                                                            *EXAMPLE 1-32  0.30    0.1      0.75  *1.0                                    Nd37 Tb3 Fe20 Co40                                                            ______________________________________                                    

In Table 4 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

                  TABLE 5                                                         ______________________________________                                        SAMP No. 1st & 3rd MAG                                                        LAYER COMP     θs                                                                              Hcs      θm                                                                            Hcm                                     (at %)         (degree)                                                                              (kOe)    (degree)                                                                            (kOe)                                   ______________________________________                                        *EXAMPLE 1-33  0.20    0.3      *0.52 2.4                                     Nd37 Tb10 Fe13 Co40                                                           *EXAMPLE 1-34  0.18    0.5      *0.57 4.0                                     Nd37 Tb15 Fe8 Co40                                                            *EXAMPLE 1-35  0.12    1.1      *0.53 6.5                                     Nd35 Tb17 Fe8 Co40                                                            *EXAMPLE 1-36  0.22    1.5      *0.58 9.4                                     Nd23 Tb17 Fe20 Co40                                                           *EXAMPLE 1-37  0.18    1.1      *0.59 6.8                                     Nd10 Tb17 Fe43 Co40                                                           *EXAMPLE 1-38  0.18    0.6      *0.52 3.9                                     Nd5 Tb15 Fe40 Co40                                                            *EXAMPLE 1-39  0.20    0.3      *0.56 1.2                                     Nd5 Tb10 Fe45 Co40                                                            *EXAMPLE 1-40  0.22    0.1      0.76  *0.5                                    Nd5 Tb3 Fe52 Co40                                                             ______________________________________                                    

In Table 5 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

In Tables 1 to 5, the Kerr rotational angle θm in the laser wavelengthof 500 nm measured from the substrate side and the coercivity Hcm areboth listed for each composition of the 40 examples. On the other hand,with respect to the comparative example shown in FIG. 2, the Kerrrotational angle θm in the laser wavelength of 500 nm measured from thesubstrate side is 0.60° and the coercivity Hcm is 11.8 kOe.

Tables 1 to 5 indicate the following test results: In the examples 1-1to 1-28, the Kerr rotational angle is larger than that of thecomparative example, and in addition the coercivity is 2 kOe or more.Here, the coercivity of 2 kOe is the minimum coercivity value requiredto stably maintain the magnetization switching domains for holdingrecording information. In other words, if the coercivity of themagnetization switching domains is less than 2 kOe, the domains areunstable, so that the S/N ratio in the reproduced signal is 45 dB orless, thus degrading the reliability as the information recordingmedium. Accordingly, when the composition as in the examples 1-1 to 1-28is adopted as the first and third magnetic layers 11 and 13respectively, it is possible to provide a magneto-optical recordingmedium high in the S/N ratio in the short wavelength range and stable inthe magnetic recording domains. In general, when the first and thirdmagnetic layers 11 and 13 are formed of the light rare earthelement--heavy rare earth element--transition metal alloy (where thelight rare earth element is Nd or Pr), it is preferable to satisfy thefollowing conditions:

    10≦x≦35 (at %)

and

    1≦y≦15 (at %)

where x (at %) denotes the composition ratio of the light rare earthelement; and y (at %) denotes the composition ratio of the heavy rareearth element.

The other evaluation test results with respect to the embodiment 1 shownin FIG. 1 will be described hereinbelow.

In this test, the composition of the first and third magnetic layers 11and 13 was fixed to Nd25Tb7Fe33Co35 (at %), and 8 examples of thecomposition of the second magnetic layer 12 were prepared as listed inTable 6. Here, the prepared examples are referred to as the embodiments1-41 to 1-48. In Table 6, the Kerr rotational angle θs in the wavelength500 nm and the coercivity Hcs of the second magnetic layers of singlelayer structure are both listed for each composition of 8 examples.Further, in the above-mentioned composition of the first and thirdmagnetic layers 11 and 13 of single layer structure, the Kerr rotationalangle is 0.34° in the wavelength of 500 nm and the coercivity is 0.8kOe.

                  TABLE 6                                                         ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 1-41     0.24    10.8    1.15  5.8                                    Nd8 Tb21 Fe53 Co18                                                            EXAMPLE 1-42     0.23    9.7     1.18  6.0                                    Nd8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 1-43     0.24    11.4    1.16  7.1                                    Nd8 Tb16 Dy5 Fe53 Co18                                                        EXAMPLE 1-44     0.25    10.4    1.19  6.8                                    Nd8 Dy22 Fe53 Co19                                                            EXAMPLE 1-45     0.25    8.2     1.15  5.6                                    Pr8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 1-46     0.24    8.5     1.18  6.0                                    Pr8 Tb16 Dy5 Fe53 Co18                                                        EXAMPLE 1-47     0.19    12.3    0.75  8.3                                    Tb22 Fe73 Co5                                                                 EXAMPLE 1-48     0.18    11.5    0.77  9.0                                    Tb22 Fe67 Co6 Cr5                                                             ______________________________________                                    

In Table 6, the Kerr rotational angle θm in the laser wavelength of 500nm measured on the substrate side and the coercivity Hcm are both listedfor composition of the examples 1-41 to 1-48. Table 6 indicates thefollowing test results: In the examples 1-41 to 1-48, the Kerrrotational angle is larger than that of the comparative example, and inaddition the coercivity is 2 kOe or more. Therefore, in these examples,it is possible to provide the magneto-optical recording medium high inthe S/N ratio in the short wavelength range and stable in the recordingmagnetic domains. In addition, Table 6 indicates that it is preferablethat the composition includes light rare earth element Nd or Pr(examples 1-41 to 1-46), because a larger Kerr rotational angle can beobtained, in comparison with the composition including only heavy rareearth element Tb as the rare earth elements (examples 1-47 and 1-48).

In general, with respect to the layer thickness ratios of the threemagnetic layers 11, 12 and 13, the enhancement of the Kerr rotationalangle of the whole recording film 17 increases with increasing layerthickness ratio of the layers (i.e., first and third magnetic layers 11and 13) having a large Kerr rotational angle, and the coercivity of thewhole recording film 17 increases with increasing layer thickness ratioof the layer (i.e., second magnetic layer 12) having a large coercivity.Therefore, the optimum layer thickness ratios are selected so that theenhancement of the kerr rotational angle and the coercivity of the wholerecording film 17 can be determined to an appropriate value,respectively.

The film thickness of the recording film 17 is preferably 400 angstromor less. This is because if the recording film 17 is as thin as thisextent, the light can be transmitted therethrough sufficiently, so thatit is possible to utilize the advantage of adopting the reflectionstructure.

The test with respect to the film thickness of the recording film 17 ismade as follows: A first protective layer 14 of 600 angstrom inthickness, a first magnetic layer 11 of 60 angstrom in thickness, asecond magnetic layer 12 of x angstrom in thickness, a third magneticlayer 13 of 60 angstrom in thickness, a second protective layer 15 of200 angstrom in thickness, and a reflection layer 16 of 600 angstrom inthickness are laminated on a polycarbonate substrate 10 in sequence.Examples of the second magnetic layers of x (80, 180, 230, 260 and 310,respectively) angstrom are prepared. The Kerr rotational angles θm ofthese examples are measured from the substrate side. Further, thecompositions of the respective layers of the respective samples are thesame as with the case of the embodiment 1-1. The film thicknesses ofthese samples are 200, 300, 350, 380 and 430 angstrom, respectively, andthe Kerr rotational angles θm in the wavelength 500 nm obtained whenmeasured from the sample substrate side are 1.14, 1.05, 0.86, 0.79 and0.63, respectively. Further, the coercivity Hcm of these samples are4.0, 4.8, 5.4, 5.7 and 5.9 kOe, respectively.

Further, in the embodiment 1, it is also possible to obtain the sameeffect as described above, when the composition of the first magneticlayer 11 is not necessarily the same as that of the third magnetic layer13.

In the embodiment 1, the Curie temperatures of the first and thirdmagnetic layers 11 and 13 are both 200° C. or higher.

EMBODIMENT 2 (2-LAYER SANDWICH+REFLECTION STRUCTURE)

FIG. 3 is a side cross-sectional view showing the embodiment 2. Thefeature of this embodiment is that the recording film is formed by twomagnetic layers of sandwiched structure and further the reflectionstructure is incorporated. That is, a first protective layer 14 of 600angstrom in thickness, a first magnetic layer 11 of 80 angstrom inthickness, a second magnetic layer 12 of 120 angstrom in thickness, asecond protective layer 15 of 200 angstrom in thickness, and areflection layer 16 of 600 angstrom in thickness are laminated on apolycarbonate substrate 10 in sequence by magnetron sputtering. Thematerial of the first and second protective layers 14 and 15 is AlSiN;and that of the reflection layer 16 is Al. Further, the material of thefirst and second magnetic layers 11 and 12 is a light rare earthelement--heavy rare earth element--transition metal alloy in the sameway as in the embodiment 1. The first magnetic layer 11 includes acomposition of the light rare earth major range, and the second magneticlayer 12 includes a composition of the heavy rare earth major range.Further, the first and second magnetic layers 11 and 12 are exchangecoupled to each other to constitute the recording layer 18.

The evaluation test results of the embodiment 2 shown in FIG. 3 will bedescribed hereinbelow.

FIG. 2 is a side cross-sectional view showing a comparative example usedfor the evaluation test. A first protective layer 14 of 600 angstrom inthickness, a recording film 12' of single layer structure with athickness of 200 angstrom, a second protective layer 15 of 200 angstromin thickness, and a reflection layer 16 of 600 angstrom in thickness arelaminated on a polycarbonate substrate 10 in sequence by magnetronsputtering. The material of the protective layers 14 and 15 is AlSiN,and that of the reflection layer 16 is Al. The recording film 12' is ofan alloy Nd6Dy22Fe52Co20 (at %) of heavy rare earth major rangecomposition. In the single layer of the recording film 12', the Kerrrotational angle in the laser wavelength of 500 nm is 0.25° and thecoercivity is 10.5 kOe.

With respect to the embodiment 2 as shown in FIG. 3, the composition ofthe second magnetic layer 12 is the same as that Nd6Dy22Fe52Co20 (at %)of the recording film 12' of the comparative example. Further, withrespect to the first magnetic layer 11, 40 samples of differentcomposition as listed in Tables 7, 8, 9, 10 and 11 were prepared. Thesesamples are referred to as the example 1-1 to example 2-40 hereinafter.Further, in Tables 7 to 11, the Kerr rotational angle θs in the laserwavelength of 500 nm and the coercivity Hcs of the second magnetic layerof single layer structure are both listed for each composition of 40examples.

                  TABLE 7                                                         ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 2-1      0.37    0.3     1.05  4.2                                    Nd15 Tb5 Fe45 Co35                                                            EXAMPLE 2-2      0.33    0.4     1.00  5.3                                    Nd15 Tb8 Fe42 Co35                                                            EXAMPLE 2-3      0.32    0.6     0.90  6.0                                    Nd15 Tb10 Fe40 Co35                                                           EXAMPLE 2-4      0.39    0.4     1.10  4.6                                    Nd23 Tb5 Fe34 Co38                                                            EXAMPLE 2-5      0.36    0.6     1.15  6.1                                    Nd23 Tb8 Fe31 Co38                                                            EXAMPLE 2-6      0.35    0.8     1.00  6.4                                    Nd23 Tb10 Fe29 Co38                                                           EYAMPLE 2-7      0.38    0.3     1.05  4.1                                    Nd30 Tb5 Fe25 Co40                                                            EXAMPLE 2-8      0.33    0.4     1.00  4.9                                    Nd30 Tb8 Fe22 Co40                                                            ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        SAMP No. 1st MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 2-9      0.32    0.6     0.88  5.5                                    Nd30 Tb10 Fe20 Co40                                                           EXAMPLE 2-10     0.35    0.6     1.05  5.9                                    Nd23 Dy8 Fe29 Co40                                                            EXAMPLE 2-11     0.35    0.7     1.11  6.2                                    Nd23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 2-12     0.36    0.3     0.95  4.3                                    Pr15 Tb5 Fe40 Co40                                                            EXAMPLE 2-13     0.36    0.7     1.00  5.7                                    Pr23 Tb8 Fe29 Co40                                                            EXAMPLE 2-14     0.32    0.4     0.98  4.9                                    Pr30 Tb8 Fe30 Co30                                                            EXAMPLE 2-15     0.35    0.6     1.06  5.5                                    Pr23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 2-16     0.35    0.4     1.04  5.7                                    Pr23 Dy8 Fe29 Co40                                                            ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        SAMP No. 1st MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 2-17     0.26    0.2     0.90  2.9                                    Nd10 Tb3 Fe47 Co40                                                            EXAMPLE 2-18     0.37    0.2     1.10  3.3                                    Nd15 Tb3 Fe42 Co40                                                            EXAMPLE 2-19     0.39    0.2     1.15  3.3                                    Nd23 Tb3 Fe34 Co40                                                            EXAMPLE 2-20     0.39    0.2     1.10  3.1                                    Nd30 Tb3 Fe27 Co40                                                            EXAMPLE 2-21     0.38    0.2     0.80  2.4                                    Nd35 Tb3 Fe22 Co40                                                            EXAMPLE 2-22     0.28    0.4     0.70  4.0                                    Nd35 Tb10 Fe15 Co40                                                           EXAMPLE 2-23     0.26    0.8     0.68  5.2                                    Nd35 Tb15 Fe10 Co40                                                           EXAMPLE 2-24     0.28    1.0     0.70  7.8                                    Nd30 Tb15 Fe15 Co40                                                           ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        SAMP No. 1st MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 2-25     0.29    1.2     0.75  8.6                                    Nd23 Tb15 Fe22 Co40                                                           EXAMPLE 2-26     0.27    1.0     0.72  7.3                                    Nd15 Tb15 Fe30 Co40                                                           EXAMPLE 2-27     0.26    0.8     0.69  5.6                                    Nd10 Tb15 Fe35 Co40                                                           EXAMPLE 2-28     0.29    0.4     0.80  4.0                                    Nd10 Tb10 Fe40 Co40                                                           *EXAMPLE 2-29    0.22    0.05    0.90  *0.6                                   Nd10 Tb1 Fe49 Co40                                                            *EXAMPLE 2-30    0.40    0.05    1.20  *1.0                                   Nd23 Tb1 Fe36 Co40                                                            *EXAMPLE 2-31    0.37    0.05    0.68  *0.8                                   Nd35 Tb1 Fe24 Co40                                                            *EXAMPLE 2-32    0.30    0.1     0.65  *1.1                                   Nd37 Tb3 Fe20 Co40                                                            ______________________________________                                    

In Table 10 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

                  TABLE 11                                                        ______________________________________                                        SAMP No. 1st MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        *EXAMPLE 2-33    0.20    0.3     *0.49 2.5                                    Nd37 Tb10 Fe13 Co40                                                           *EXAMPLE 2-34    0.18    0.5     *0.55 3.9                                    Nd37 Tb15 Fe8 Co40                                                            *EXAMPLE 2-35    0.12    1.1     *0.50 6.4                                    Nd35 Tb17 Fe8 Co40                                                            *EXAMPLE 2-36    0.22    1.5     *0.55 9.9                                    Nd23 Tb17 Fe20 Co40                                                           *EXAMPLE 2-37    0.18    1.1     *0.50 7.0                                    Nd10 Tb17 Fe43 Co40                                                           *EXAMPLE 2-38    0.18    0.6     *0.49 4.1                                    Nd5 Tb15 Fe40 Co40                                                            *EXAMPLE 2-39    0.20    0.3     *0.55 1.5                                    Nd5 Tb10 Fe45 Co40                                                            *EXAMPLE 2-40    0.22    0.1     0.75  *0.4                                   Nd5 Tb3 Fe52 Co40                                                             ______________________________________                                    

In Table 11 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

In Tables 7 to 11, the Kerr rotational angle θm in the laser wavelengthof 500 nm measured from the substrate side and the coercivity Hcm areboth listed for each composition of the 40 examples 2-1 to 2-40. On theother hand, with respect to the comparative example shown in FIG. 2, theKerr rotational angle θm obtained when measured from the substrate sidein the laser wavelength of 500 nm is 0.60° and the coercivity Hcm is11.8 kOe.

Tables 7 to 11 indicate the following test results: In the examples 2-1to 2-28, the Kerr rotational angle is larger than that of thecomparative example, and in addition the coercivity is 2 kOe or more.Accordingly, it is possible to provide a magneto-optical recordingmedium high in the S/N ratio in the short wavelength range and stable inthe magnetic recording domains. In general, when the first magneticlayer 11 is formed of the light rare earth element--heavy rare earthelement--transition metal alloy (where the light rare earth element isNd or Pr), it is preferable to satisfy the following conditions:

    10≦x≦35 (at %)

and

    1≦y≦15 (at %)

where x (at %) denotes the composition ratio of the light rare earthelement; and y (at %) denotes the composition ratio of the heavy rareearth element.

The other evaluation test results with respect to the embodiment 2 shownwill be described hereinbelow.

In this test, the composition of the first magnetic layer 11 was fixedto Nd25Tb7Fe33Co35 (at %), and 8 examples of the composition (the sameas in Table 6) of the second magnetic layer 12 were prepared as listedin Table 12. Here, the prepared examples are referred to as the examples2-41 to 2-48. In Table 12, the Kerr rotational angle θs in thewavelength 500 nm and the coercivity Hcs of the second magnetic layersof single layer structure are both listed for each composition of 8examples. Further, in the composition of the first magnetic layer 11 ofsingle layer structure, the Kerr rotational angle in the wavelength of500 mm is 0.34° and the coercivity is 0.8 kOe.

                  TABLE 12                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 2-41     0.24    10.8    1.06  6.1                                    Nd8 Tb21 Fe53 Co18                                                            EXAMPLE 2-42     0.23    9.7     1.12  5.9                                    Nd8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 2-43     0.42    11.4    1.09  7.0                                    Nd8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 2-44     0.25    10.4    1.06  6.8                                    Nd8 Dy22 Fe53 Co19                                                            EXAMPLE 2-45     0.52    8.2     1.10  5.7                                    Pr8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 2-46     0.24    8.5     1.12  5.8                                    Pr8 Tb16 Dy5 Fe53 Co18                                                        EXAMPLE 2-47     0.19    12.3    0.69  8.3                                    Tb22 Fe73 Co5                                                                 EXAMPLE 2-48     0.18    11.5    0.71  8.4                                    Tb22 Fe67 Co6 Cr5                                                             ______________________________________                                    

In Table 12, the Kerr rotational angle θm measured from the substrateside in the laser wavelength of 500 nm and the coercivity Hcm are bothlisted for composition of the examples 2-41 to 2-48. Table 12 indicatesthe following test results: In the examples 2-41 to 2-48, the Kerrrotational angle is larger than that of the comparative example, and inaddition the coercivity is 2 kOe or more. Therefore, in these examples2-41 to 2-48, it is possible to provide the magneto-optical recordingmedium high in the S/N ratio in the short wavelength range and stable inthe recording magnetic domains. In addition, Table 12 indicates that itis possible to obtain a larger Kerr rotational angle when the secondmagnetic layer 12 is a perpendicular magnetized film including the lightrare earth element such as Nd or Pr, as compared with the perpendicularmagnetized film including the heavy rare earth element such as Tb as therare earth element.

The film thickness ratio of the first and second magnetic layers 11 and12 is so selected that the enhancement of the Kerr rotational angle andthe coercivity of the whole recording film 18 can be determined as anappropriate value, respectively.

The film thickness of the recording film 18 is preferably 400 angstromor less to utilize the advantage of the reflection structure. Withrespect to this point, the inventors performed the followingexperiments: A protective layer 14 of 600 angstrom in thickness, a firstmagnetic layer 11 of 60 angstrom in thickness, a second magnetic layer12 of x angstrom in thickness, a second protective layer 15 of 200angstrom in thickness, and a reflection layer 16 of 600 angstrom inthickness are laminated on a polycarbonate substrate 10 in sequence.Examples of the second magnetic layers of x (120, 220, 270, 300 and 350,respectively) angstrom are prepared. The Kerr rotational angles θm ofthese examples are measured. Further, the compositions of the respectivelayers of the respective samples are the same as with the case of theexample 1-1. The film thicknesses of the recording films are 200, 300,350, 380 and 430 angstrom, respectively, and the Kerr rotational anglesθm in the wavelength 500 nm obtained when measured from the samplesubstrate side are 1.05, 0.93, 0.81, 0.72 and 0.61, respectively.Further, the coercivity Hcm of these samples are 4.2, 5.2, 5.6, 5.9 and6.1 kOe, respectively. In the embodiment 2, the Curie temperatures ofthe first magnetic layer 11 is 200° C. or higher.

Further, the recording film 18 of two layer sandwich structure in theembodiment 2 can be also utilized as the reproducing film (the magneticfilm located the nearest to the laser pickup) as the recording mediumfor overwriting (exchange coupled with another magnetic film) or thereproducing film as the magnetic induced super resolution recordingmedium.

EMBODIMENT 3 (MAGNETIC INDUCED SUPER RESOLUTION)

In this embodiment 3, the recording film 18 of two layer sandwichstructure in the embodiment 2 is utilized as the reproducing film forthe magnetic induced super resolution recording medium.

FIG. 4 is a side cross-sectional view showing the embodiment 3. A firstprotective layer 14 of 600 angstrom in thickness, a first magnetic layer31 of 150 angstrom in thickness, a second magnetic layer 32 of 350angstrom in thickness, a third magnetic layer 33 of 100 angstrom inthickness, a fourth magnetic layer 34 of 200 angstrom in thickness, afifth magnetic layer 35 of 500 angstrom in thickness, and a secondprotective layer 15 of 800 angstrom in thickness are laminated on apolycarbonate substrate 10 in sequence by magnetron sputtering. Thematerial of the first and second protective layers 14 and 15 is AlSiN.The first and second magnetic layers 31 and 32 constitute a reproducingfilm 17 for the magnetic induced super resolution medium of thisembodiment. Further, the material of the first and second magneticlayers 31 and 32 is a light rare earth element--heavy rare earthelement--transition metal alloy including composition of both the lightrare earth major range and heavy rare earth major range, respectively inthe same way as in the first and second magnetic layers 11 and 12 of theembodiment 2 as shown in FIG. 3. The third magnetic layer 33 functionsas an auxiliary reproducing film for controlling transcription process.The fourth magnetic layer 34 functions as a control film for controllingthe interface wall. The fifth magnetic layer 35 functions as a recordingfilm for storing recorded information.

The evaluation test results of the embodiment 3 shown in FIG. 4 will bedescribed hereinbelow.

Table 13 lists the compositions of the magnetic layers 31 to 35 of asample related to the embodiment 3 used for this test and the physicalproperties of the magnetic layers of single structure having the samecomposition. These samples are referred to as an example 3-1.

                  TABLE 13                                                        ______________________________________                                        RESP MAG LAYER COMP                                                                           θs                                                                              Hcs      Tc                                           (at %)          (degree)                                                                              (kOe)    (°C.)                                 ______________________________________                                        MAG LAYER 31    0.35    0.6      280  TM                                      Nd23 Tb8 Fe31 Co38                                                            MAG LAYER 32    0.24    2.1      240  TM                                      Nd6 Dy16 Tb3 Fe65 Co10                                                        MAG LAYER 33    --      9.8      145  TM                                      Nd6 Dy22 Fe66 Co7                                                             MAG LAYER 34    --      0.4      250  TM                                      Nd6 Gd21 Fe66 Co7                                                             MAG LAYER 35    --      12.0     240  RE                                      Dy15 Tb10 Fe47 Co28                                                           ______________________________________                                         TM: Transition metal rich                                                     RE: Rare earth element rich                                              

FIG. 5 is a side cross-sectional view showing a comparative example usedfor this evaluation test. A first protective layer 14, a reproducingfilm 32', a third magnetic film (auxiliary reproducing film) 33, afourth magnetic layer (control film) 34, a fifth magnetic layer(recording film) 35, and a second protective layer 15 are laminated on atransparent substrate 10 in sequence by magnetron sputtering. The filmthickness and the material of the layers are the same as those of theembodiment 3, except the reproducing film 32'. With respect to thereproducing film 32', the film thickness is 500 angstrom in the same wayas that of the reproducing film 37 of the example 3-1, and thecomposition is the same as with the case of the second magnetic layer32.

The Kerr rotational angle of the example 3-1 is 0.67°, and that of thecomparative example is 0.45° both in the wavelength of 500 nm whenmeasured from the substrate side. This indicates that the example 3-1 ismore advantageous in the recording and reproducing characteristics thanthe comparative example.

Further, the example 3-1 and the comparative example were tested withrespect to the recording and reproducing operation in accordance withthe following specification; The laser beam is a SHG light beam of 532nm generated by a semiconductor exited Nd-YAG laser and obtained througha KTiOPO4 element. The diameter of the laser spot is 0.8 μm. A 45 degreedifferential detection method is adopted, and a PIN photodiode is usedas an optical detector. Information signals of 7.0 MHz are written inthe example 3-1 and the comparative example at a linear speed of 5.7m/sec. The initialization magnetizing field is 3.0 kOe, which isopposite to the recording magnetizing field in the direction. Under theabove-mentioned specifications, the signals are reproduced in accordancewith the magnetic induced super resolution method. The pit error duringthe signal reproduction is 1.2×10⁻⁴ in the example 3-1 and 2.1×10⁻¹ inthe comparative example, which clearly indicates that the example 3-1 issuperior to the comparative example. This is because the Kerr rotationalangle of the reproducing film 32' of the comparative example is notsufficiently large in the short wavelength range.

Further, with respect to the above embodiment 3, the same test was madeby preparing the sample in which the material Nd of the first and secondlayers 31 and 32 of the example 3-1 was replaced with Pr. The example isreferred to as an example 3-2. Table 14 lists the compositions of therespective layers and the physical properties of the layers of singlelayer including the same composition.

                  TABLE 14                                                        ______________________________________                                        RESP MAG LAYER COMP                                                                           θs                                                                              Hcs      Tc                                           (at %)          (degree)                                                                              (kOe)    (°C.)                                 ______________________________________                                        MAG LAYER 31    0.34    0.7     265   TM                                      Pr23 Tb8 Fe31 Co38                                                            MAG LAYER 32    0.23    1.8     235   TM                                      Pr6 Dy16 Tb3 Fe65 Co10                                                        MAG LAYER 33            2.1     140   TM                                      Nd6 Dy22 Fe66 Co7                                                             MAG LAYER 34            0.5     240   TM                                      Pr6 Gd21 Fe66 Co7                                                             MAG LAYER 35            12.0    240   RE                                      Dy15 Tb10 Fe47 Co28                                                           ______________________________________                                         TM: Transition metal rich                                                     RE: Rare earth element rich                                              

In the example 3-2, the Kerr rotational angle θm is 0.66° in thewavelength of 500 nm when measured from the substrate side. Further, thebit error rate during the reproduction in accordance with the magneticinduced super resolution method under the above-mentioned specificationis 1.8×10⁻⁴. The above-mentioned results indicate that the example 3-2is also advantageous in comparison with the comparative example.

With respect to the composition of the first magnetic layer 31 of theembodiment 3, in general, the following relationship is preferable fromthe standpoint of recording and reproducing characteristics:

    10≦x≦35 (at %)

and

    1≦y≦15 (at %)

where x (at %) denotes the composition ratio of the light rare earthelement; and y (at %) denotes the composition ratio of the heavy rareearth element.

Further, another example formed by the respective layers having the samecompositions (Table 13) as the example 3-1 but different thickness fromthe example 3-1 was prepared. This example is referred to as example3-3. In this example 3-3, the thickness of the respective layer is 700angstrom in the first protective layer 14; 100 angstrom in the firstmagnetic layer 31; 200 angstrom in the second magnetic layer 32; 150angstrom in the third magnetic layer 33; 500 angstrom in the fourthmagnetic layer 34; and 800 angstrom in the second protective layer 15.As the results of the reproducing test in accordance with the magneticinduced super resolution method under the above-mentionedspecifications, the bit error rate is 2.0×10⁻⁴. The above-mentionedresults indicate that the example 3-3 is also advantageous in comparisonwith the comparative example.

EMBODIMENT 4

                  TABLE 15                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 4-1      0.37    0.3     0.96  5.7                                    Nd15 Tb5 Fe45 Co35                                                            EXAMPLE 4-2      0.33    0.4     0.95  7.2                                    Nd15 Tb8 Fe42 Co35                                                            EXAMPLE 4-3      0.32    0.6     0.86  8.3                                    Nd15 Tb10 Fe40 Co35                                                           EXAMPLE 4-4      0.39    0.4     1.02  6.2                                    Nd23 Tb5 Fe34 Co38                                                            EXAMPLE 4-5      0.36    0.6     1.00  8.4                                    Nd23 Tb8 Fe31 Co38                                                            EXAMPLE 4-6      0.35    0.8     0.95  9.8                                    Nd23 Tb10 Fe29 Co38                                                           EXAMPLE 4-7      0.38    0.3     0.98  5.6                                    Nd30 Tb5 Fe25 Co40                                                            EXAMPLE 4-8      0.33    0.4     0.95  7.0                                    Nd30 Tb8 Fe22 Co40                                                            ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 4-9      0.32    0.6     0.81  8.5                                    Nd30 Tb10 Fe20 Co40                                                           EXAMPLE 4-10     0.35    0.6     0.96  7.5                                    Nd23 Dy8 Fe29 Co40                                                            EXAMPLE 4-11     0.35    0.7     0.98  7.9                                    Nd23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 4-12     0.36    0.3     0.89  6.8                                    Pr15 Tb5 Fe40 Co40                                                            EXAMPLE 4-13     0.36    0.7     0.92  6.9                                    Pr23 Tb8 Fe29 Co40                                                            EXAMPLE 4-14     0.32    0.4     0.88  5.9                                    Pr30 Tb8 Fe30 Co30                                                            EXAMPLE 4-15     0.35    0.6     0.95  6.3                                    Pr23 Tb5 Dy3 Fe29 Co40                                                        EXAMPLE 4-16     0.35    0.4     0.94  5.7                                    Pr23 Dy8 Fe29 Co40                                                            ______________________________________                                    

                  TABLE 17                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 4-17     0.26    0.2     0.81  2.9                                    Nd10 Tb3 Fe47 Co40                                                            EXAMPLE 4-18     0.37    0.2     1.02  3.5                                    Nd15 Tb3 Fe42 Co40                                                            EXAMPLE 4-19     0.39    0.2     1.02  3.9                                    Nd23 Tb5 Fe34 Co40                                                            EXAMPLE 4-20     0.39    0.2     1.02  3.4                                    Nd30 Tb3 Fe27 Co40                                                            EXAMPLE 4-21     0.38    0.2     0.76  2.9                                    Nd35 Tb3 Fe22 Co40                                                            EXAMPLE 4-22     0.28    0.4     0.66  5.3                                    Nd35 Tb10 Fe15 Co40                                                           EXAMPLE 4-23     0.26    0.8     0.61  7.4                                    Nd35 Tb15 Fe10 Co40                                                           EXAMPLE 4-24     0.28    1.0     0.68  9.3                                    Nd30 Tb15 Fe15 Co40                                                           ______________________________________                                    

                  TABLE 18                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 4-25     0.29    1.2     0.71  11.5                                   Nd23 Tb15 Fe22 Co40                                                           EXAMPLE 4-26     0.27    1.0     0.69  9.4                                    Nd15 Tb15 Fe30 Co40                                                           EXAMPLE 4-27     0.26    0.8     0.69  7.9                                    Nd10 Tb15 Fe35 Co40                                                           EXAMPLE 4-28     0.29    0.4     0.78  6.8                                    Nd10 Tb10 Fe40 Co40                                                           *EXAMPLE 4-29    0.22    0.05    0.86  *0.6                                   Nd10 Tb1 Fe49 Co40                                                            *EXAMPLE 4-30    0.40    0.05    1.00  *1.3                                   Nd23 Tb1 Fe36 Co40                                                            *EXAMPLE 4-31    0.37    0.05    0.66  *0.7                                   Nd35 Tb1 Fe24 Co40                                                            *EXAMPLE 4-32    0.30    0.1     0.65  *0.9                                   Nd37 Tb3 Fe20 Co40                                                            ______________________________________                                    

In Table 18 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

                  TABLE 19                                                        ______________________________________                                        SAMP No. 2nd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        *EXAMPLE 4-33    0.20    0.3     *0.45 2.3                                    Nd37 Tb10 Fe13 Co40                                                           *EXAMPLE 4-34    0.18    0.5     *0.51 3.9                                    Nd37 Tb15 Fe8 Co40                                                            *EXAMPLE 4-35    0.12    1.1     *0.48 8.1                                    Nd35 Tb17 Fe8 Co40                                                            *EXAMPLE 4-36    0.22    1.5     *0.52 12.3                                   Nd23 Tb17 Fe20 Co40                                                           *EXAMPLE 4-37    0.18    1.1     *0.49 9.0                                    Nd10 Tb17 Fe43 Co40                                                           *EXAMPLE 4-38    0.18    0.6     *0.46 8.1                                    Nd5 Tb15 Fe40 Co40                                                            *EXAMPLE 4-39    0.20    0.3     *0.50 4.9                                    Nd5 Tb10 Fe45 Co40                                                            *EXAMPLE 4-40    0.22    0.1     0.71  *0.5                                   Nd5 Tb3 Fe52 Co40                                                             ______________________________________                                    

In Table 19 above, the asterisk * indicates the example which satisfiesthe following conditions:

    θm<0.6° or Hcm<2.0 kOe

EMBODIMENT 4 (3-LAYER SANDWICH+REFLECTION STRUCTURE)

FIG. 6 is a side cross-sectional view showing the embodiment 4. A firstprotective layer 14 of 600 angstrom in thickness, a first magnetic layer41 of 60 angstrom in thickness, a second magnetic layer 42 of 80angstrom in thickness, a third magnetic layer 43 of 60 angstrom inthickness, a second protective layer 15 of 200 angstrom in thickness,and a reflection layer 16 of 600 angstrom in thickness are laminated ona polycarbonate substrate 10 in sequence by magnetron sputtering. Thematerial of the first and second protective layers 14 and 15 is AlSiN;and that of the reflection layer 16 is Ag of 200 angstrom thickness andAl96Ti4 (at %) of 400 angstrom thickness both laminated in thissequence. Further, the material of the first and third magnetic layers41 and 43 is a light rare earth element--heavy rare earthelement--transition metal alloy of the heavy rare earth major rangecomposition. The second magnetic layer 42 is a light rare earthelement--heavy rare earth element--transition metal alloy of the lightrare earth major range composition.

The evaluation test results of the embodiment 4 will be describedhereinbelow.

FIG. 7 is a side cross-sectional view showing a comparative example usedfor the evaluation test. A first protective layer 14 of 600 angstrom inthickness, a recording film 41' of single layer structure with athickness of 200 angstrom, a second protective layer 15 of 200 angstromin thickness, and a reflection layer 16 of 600 angstrom in thickness arelaminated on a polycarbonate substrate 10 in sequence by magnetronsputtering. The material of the protective layers 14 and 15 is ALSiN,and that of the reflection layer 16 is Ag of 200 angstrom thickness andAl96Ti4 (at %) both laminated in this sequence. Further, the reflectionfactors of Ag and AlTi are 98% and 90%, respectively. Further, thematerial of the recording film 41' is an alloy of Nd6Dy22Fe52Co22 (at %)of heavy rare earth major range composition. In the single layer of therecording film 41', the Kerr rotational angle in the laser wavelength of500 nm is 0.25° and the coercivity is 10.5 kOe.

In the evaluation test of the embodiment 4, the material of the firstand third magnetic layers 41 and 43 is the same as that Nd6Dy22Fe52Co20(at %) of the heavy rare earth major range composition in the recordingfilm 41' of the comparative example. Further, with respect to the secondmagnetic layer 42, 40 samples of different composition as listed inTables 15, 16, 17, 18 and 19 were prepared. These samples are referredto as the example 4-1 to example 4-40 hereinafter. The materials of theprotective layers 14 and 15 and the reflective layer 16 of theseexamples are the same as those of the comparative example. Further, inTables 15 to 19, the Kerr rotational angle θs in the laser wavelength of500 nm and the coercivity Hcs of the magnetic layers of single layerstructure are both listed for each composition of 40 examples.

In Tables 15 to 19, the Kerr rotational angle θm measured from thesubstrate side in the laser wavelength of 500 nm and the coercivity Hcmare both listed for each composition of the examples 4-1 to 4-40. On theother hand, with respect to the comparative example, the Kerr rotationalangle θm when measured from the substrate side in the laser wavelengthof 500 nm is 0.60° and the coercivity Hcm is 11.8 kOe

Tables 15 to 19 indicate that in the examples 4-1 to 4-28, the Kerrrotational angle is larger than that of the comparative example, and inaddition the coercivity is 2 kOe or more, so that the S/N ratio is highin the short wavelength range and it is possible to provide amagneto-optical recording medium stable in the magnetic recordingdomains. In general, when the second magnetic layer 42 is formed of thelight rare earth element--heavy rare earth element--transition metalalloy (where the light rare earth element is Nd or Pr), it is preferableto satisfy the following conditions:

    10≦x≦35 (at %)

and

    1≦y≦15 (at %)

where x (at %) denotes the composition ratio of the light rare earthelement; and y (at %) denotes the composition ratio of the heavy rareearth element.

The other evaluation test results will be described hereinbelow.

In this test related to the embodiment 4, the composition of the secondmagnetic layer 42 was fixed to Nd25Tb7Fe33Co35 (at %), and 8 exampleshaving the first and third magnetic layers 41 and 43 of differentcomposition respectively were prepared as listed in Table 20. Here, theprepared examples are referred to as the embodiments 4-41 to 4-48. InTable 20, the Kerr rotational angle θs in the wavelength 500 nm and thecoercivity Hcs of the respective first and third magnetic layers ofsingle layer structure are both listed for each composition of theabove-mentioned 8 examples. Further, in the second magnetic layer 42 ofsingle layer structure, the Kerr rotational angle is 0.34° in thewavelength 500 nm and the coercivity is 0.8 kOe.

                  TABLE 20                                                        ______________________________________                                        SAMP No. 3rd MAG LAYER                                                        COMP             θs                                                                              Hcs     θm                                                                            Hcm                                    (at %)           (degree)                                                                              (kOe)   (degree)                                                                            (kOe)                                  ______________________________________                                        EXAMPLE 4-41     0.24    10.8    1.00  8.3                                    Nd8 Tb21 Fe53 Co18                                                            EXAMPLE 4-42     0.23    9.7     1.05  7.9                                    Nd8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 4-43     0.24    11.4    0.98  7.8                                    Nd8 Tb16 Dy5 Fe53 Co18                                                        EXAMPLE 4-44     0.25    10.4    0.96  8.2                                    Nd8 Dy22 Fe53 Co19                                                            EXAMPLE 4-45     0.25    8.2     1.02  8.3                                    Pr8 Dy16 Tb5 Fe53 Co18                                                        EXAMPLE 4-46     0.24    8.5     1.03  8.0                                    Pr8 Tb16 Dy5 Fe53 Co18                                                        EXAMPLE 4-47     0.19    12.3    0.66  10.5                                   Tb22 Fe73 Co5                                                                 EXAMPLE 4-48     0.18    11.5    0.65  11.0                                   Tb22 Fe67 Co6 Cr5                                                             ______________________________________                                    

In Table 20, the Kerr rotational angle θm measured from the substrateside in the laser wavelength of 500 nm and the coercivity Hcm are bothlisted for each composition of the respective examples. Table 20indicates that in the examples 4-41 to 4-48, the Kerr rotational angleis larger than that of the comparative example, and in addition thecoercivity is 2 kOe or more, so that the S/N ratio is high in the shortwavelength range. Therefore, in these examples, it is possible toprovide the magneto-optical recording medium stable in the recordingmagnetic domains. In addition, Table 20 indicates that it is possible toobtain a larger Kerr rotational angle when the first and third magneticlayers 41 and 43 are of perpendicular magnetized film including onlylight rare earth element such as Nd or Pr, as compared with when themagnetic layers 41 and 43 are of perpendicular magnetized film includingonly heavy rare earth element such as Tb as the rare earth element.

Further, in the embodiment as shown in FIG. 6, it is possible to obtainthe same effects as described above, even if the first and thirdmagnetic layers 41 and 43 are not necessarily the same in compositionwith respect to each other. The test results which can verify the abovefact will be described hereinbelow.

Table 21 lists the composition examples of the first and third magneticlayers 41 and 43 adopted in this test, which are referred to ascomposition 1 to composition 4. Further, in Table 21, the Kerrrotational angle in the wavelength of 500 nm and the coercivity of themagnetic layers of single layer structure are shown for each compositionexample. In these composition examples, the composition 1 is acomposition example provided with the condition that the transitionmetal (TM) is rich (the composition in which the sublattice 35magnetization of the transition metal is prominent at room temperature)and further away from the compensation composition (the composition inwhich the sublattice magnetization of the transition metal is in balancewith that of the rare earth metal). The composition 2 is a compositionexample provided with the condition that the transition metal (TM) isrich and further near the compensation composition. The composition 3 isa composition example provided with the condition that the rare earthmetal (RE) is rich (the composition in which the sublatticemagnetization of the rare earth metal is prominent at room temperature)and further near the compensation composition. The composition 4 is acomposition example provided with the condition that the compositionincludes only the heavy rare earth element (e.g., Tb) as the rare earthmetal (referred to as heavy rare earth base composition, hereinafter)and further TM is rich.

                  TABLE 21                                                        ______________________________________                                        COMP            θs                                                                              Hcs      Tc                                           (at %)          (degree)                                                                              (kOe)    (°C.)                                 ______________________________________                                        COMP 1          0.25     4.0     180  TM                                      Nd6 Dy16 Tb3 Fe60 Co15                                                        COMP 2          0.23    10.5     170  TM                                      Nd6 Dy16 Tb6 Fe56 Co16                                                        COMP 3          0.20    11.4     160  RE                                      Nd6 Dy16 Tb8 Fe54 Co16                                                        COMP 4          0.18    12.3     180  TM                                      Tb22 Fe73 Co5                                                                 ______________________________________                                         TM: Transition metal rich                                                     RE: Rare earth element rich                                              

Table 22 lists the composition combinations of the first and thirdmagnetic layers 41 and 43 of the samples used for this test. Thesesamples are referred to as examples 4-49 to 4-54.

                  TABLE 22                                                        ______________________________________                                                  1st MAG LAYER 41/                                                                           θm                                                                              Hcr                                                     3rd MAG LAYER 43                                                                            (degree)                                                                              (kOe)                                         ______________________________________                                        EXAMPLE 4-49                                                                              COMP 1/COMP 1   1.16    2.1                                       EXAMPLE 4-50                                                                              COMP 1/COMP 2   1.10    4.9                                       EXAMPLE 4-51                                                                              COMP 2/COMP 2   1.04    7.5                                       EXAMPLE 4-52                                                                              COMP 2/COMP 3   0.81    10.1                                      EXAMPLE 4-53                                                                              COMP 1/COMP 3   0.91    8.6                                       EXAMPLE 4-54                                                                              COMP 1/COMP 4   0.75    12.4                                      ______________________________________                                    

Table 22 lists the Kerr rotational angle θm in the wavelength 500 nm andthe extinction field Hcr measured for the examples 4-49 to 4-54,respectively. Here, the extinction field Hcr is one index indicating thestability of the recording domain as defined below. That is, a recordingdomain with a radius of 0.25 μm is formed in a recording layer 47 inaccordance with the optical modulation recording method, and thereafteran external magnetic field is increasingly applied to the recordinglayer 47 in the direction opposite to the magnetization direction of thefirst magnetic layer 41.

In this case, an intensity of the external field increased until therecording domain is extinguished is defined as the extinction field Hcr.

As understood by Tables 21 and 22, in the combinations (examples 4-52and 4-53) such that the first magnetic layer 41 is of TM richcomposition and the third magnetic layer 43 is of RE rich composition,the Kerr rotational angle is the largest and therefore the extinctionfield is the highest, so that it is possible to provide stable recordingdomains. This is because since the TM rich composition and the RE richcomposition are opposite to each other with respect to the apparentmagnetization direction, it may be due to the fact that the energy atthe whole recording domains of the recording film 47 is low andtherefore stable, as compared with the combination of only TM richcompositions or only RE rich compositions. In addition, since the Kerrrotational angle of the TM rich composition is larger than that of theRE rich composition (in particular, the Kerr rotational angle increasesas the composition goes away from the compensation composition), theenhancement of the Kerr rotational angle can be considered as beinglarge when the first magnetic layer 41 located near the optical headside is formed of TM rich composition, as compared with the case wherethe third magnetic layer 43 located on the side of the reflection layer16 is formed of TM rich composition.

FIG. 8 shows the magnetization states of the recording domains in therecording film 47 of the above-mentioned desirable examples 4-52 and4-53, which are classified according to the apparent magnetization andthe sublattice magnetization. FIG. 8 indicates that the apparentmagnetization direction of the third magnetic layer 43 formed of the RErich composition is the same as the external magnetic field, so that thecomponent for expanding the recording domain is included in the forceapplied to the side surface wall of the recording domain.

In addition, Tables 21 and 22 indicate the following facts: in theexample 4-50, since the first and third magnetic layers 41 and 43 areboth of TM rich composition (the Kerr rotational angle is large in theshort wavelength range, as compared with the RE rich composition) andfurther the first magnetic layer 41 is away from the compensationcomposition, the Kerr rotational angle is large in the short wavelength.In addition, since the third magnetic layer 43 is of composition nearthe compensation composition, the coercivity is high. Under theseconditions, however, in the example 4-50, the Kerr rotational angle inshort wavelength is large and in addition the extinction magnetic fieldis also large, so that it is possible to provide recording medium whoserecording domains are relatively stable. Further, as with the case ofthe example 4-54, when the first magnetic layer 41 is formed of a lightrare earth element--heavy rare earth element--transition metal alloy ofTM rich composition and further the third magnetic layer 43 is formed ofa heavy rare earth element base alloy such as TbFeCo having a small Kerrrotational angle but a large coercivity, it is possible to increase thecoercivity of the recording film 47 as a whole.

The film thickness ratios of the magnetic layers 41, 42 and 43 areselected so that the enhancement of the Kerr rotational angle and thecoercivity of the whole recording film 47 can be determined as anappropriate value, respectively.

The film thickness of the recording film 47 is preferably 400 angstromor less in order to utilize the advantage of the reflection structure.The test with respect to the film thickness of the recording film 47 wasmade as follows: A first protective layer 14 of 600 angstrom inthickness, a first magnetic layer 41 of 60 angstrom in thickness, asecond magnetic layer 42 of 80 angstrom in thickness, a third magneticlayer 43 of x angstrom in thickness, a second protective layer 15 of 200angstrom in thickness, and a reflection layer 16 of 600 angstrom inthickness are laminated on a polycarbonate substrate 10 in sequence.Examples having the third magnetic layers of x (60, 160, 220, 240 and290, respectively) angstrom are prepared. The Kerr rotational angles θmof these examples are measured from the substrate side. Further, thecomposition of the respective layers of the respective samples are thesame as with the case of the embodiment 1-1. The film thicknesses ofthese samples are 200, 300, 350, 380 and 430 angstrom, respectively, andthe Kerr rotational angles θm in the wavelength 500 nm obtained whenmeasured from the sample substrate side are 0.96, 0.90, 0.81, 0.69 and0.61, respectively. Further, the coercivity Hcm of these samples are5.7, 6.5, 6.9, 7.2 and 7.3 kOe, respectively.

In the embodiment 4 as shown in FIG. 6, the Curie temperatures of thesecond magnetic layer 42 are 200° C. or higher.

The reason why the reflection layer 16 is formed by two layers of Ag andAlTi in the embodiment 4 as shown in FIG. 6 is as follows: since Ag ishigh in reflection factor in short wavelength, it is possible toincrease the reflection factor of the whole medium, and further theperformance index is increased. Further, since AlTi high in corrosionresistance and stable in reliability is laminated on the Ag layer ofhigh reflection factor for realizing a composite material, it ispossible to provide a reflection layer high in both reflection factorand reliability. Further, in this composite structure, it is possible toobtain the same effect as above, even if more economical Al (reflectionfactor: 92%) is used instead of Ag.

EMBODIMENT 5 (OVERWRITE)

In this embodiment 5, a recording film 47 of the sandwich structure asshown in FIG. 6 is applied to the recording film of optical modulationoverwrite type medium using an exchange coupled multilayer film. FIG. 9is a cross-sectional view showing this embodiment 5. In FIG. 9, a firstprotective layer 14 of 600 angstrom thickness, a first magnetic layer 51of 80 angstrom thickness, a second magnetic layer 52 of 100 angstromthickness, a third magnetic layer 53 of 300 angstrom thickness, a fourthmagnetic layer 54 of 150 angstrom thickness, a fifth magnetic layer 55of 700 angstrom thickness, and a second protective layer 15 of 800angstrom thickness are laminated in sequence on a transparent substrate10 by magnetron sputtering. The protective layers 14 and 15 are formedof AlSiN. The first, second and third magnetic layers 51, 52 and 53constitute a recording film 57, the respective compositions of which arethe same as those of the first, second and third magnetic layers of theexample 4 shown in FIG. 6. The fourth magnetic layer 54 functions as acontrol film for controlling the interface wall energy. The fifthmagnetic layer 55 functions as an auxiliary layer for holding theinitialized magnetization information.

FIG. 10 is a side cross-sectional view showing the comparative exampleused for this test. In this comparative example, the material and thefilm thickness of the respective layers are the same as those of theembodiment 5, except the recording film 53'. The composition of therecording film 53' is the same as that of the third magnetic layer 53 ofthe embodiment 5 shown in FIG. 9, and the film thickness thereof is 480angstrom.

Table 23 lists the compositions of the magnetic layers 51 to 55 of asample prepared for the embodiment 5 shown in FIG. 9 and the physicalproperties of the respective layers of single layer structure includingthe same composition. Hereinafter, the sample is referred to as anexample 5-1.

                  TABLE 23                                                        ______________________________________                                        RESP MAG LAYER COMP                                                                           θs                                                                              Hcs      Tc                                           (at %)          (degree)                                                                              (kOe)    (°C.)                                 ______________________________________                                        MAG LAYER 51    0.25    4.0      180  TM                                      Nd6 Dy16 Tb3 Fe60 Co15                                                        MAG LAYER 52    0.35    0.6      280  TM                                      Nd23 Tb8 Fe31 Co38                                                            MAG LAYER 53    0.23    11.8     140  TM                                      Nd6 Dy16 Tb6 Fe65 Co10                                                        MAG LAYER 54            0.4      250  TM                                      Nd6 Gd21 Fe66 Co7                                                             MAG LAYER 55            12.0     250  RE                                      Dy30 Fe41 Co29                                                                ______________________________________                                         TM: Transition metal rich                                                     RE: Rare earth element rich                                              

The example 5-1 and the comparative example were both measured. The Kerrrotational angle thereof are 0.65° and 0.45°, respectively.

Further, the recording and reproducing test was made to indicate thatthe example 5-1 is more advantageous than the comparative example, underthe following specifications: The laser beam is a SHG light beam of 532nm generated by a semiconductor exited Nd-YAG laser and obtained througha KTiOPO4 element. The laser beam is modulated by an AOM element. Thediameter of the laser spot is 0.8 μm. A 45 degree differential detectionmethod is adopted, and a PIN photodiode is used as an optical detector.First, a signal of 3.0 MHz is recorded, and further a signal of 5.0 MHzis overwritten. The laser power is modulated into binary of lower powerP1 and high power Ph during overwriting operation. The low power P1 andthe high power Ph are determined as follows:

    Pr<P1<Ph

where Pr denotes the laser power for reproducing the signal. Aspractical values,

Pr=1.0 mW

P1=2.7 mW

Ph=6.0 mW

The linear speed is 5.7 m/sec. The initialization magnetizing field is3.0 kOe. The recording magnetic field and the initialization magneticfield are the same in direction with respect to each other. As the testresults, the pit error during the signal reproduction is 1.0×10⁻⁵ in theexample 5-1 and 7.1×10⁻² in the comparative example, which clearlyindicates that the example 5-1 is superior to the comparative example.This is because the Kerr rotational angle of the recording film 53' ofthe comparative example is not sufficiently large in the shortwavelength range.

Further, other samples in which Nd in the compositions of the magneticlayers 51 to 54 listed in Table 23 was replaced with Pr were prepared,and the similar test was made. This sample is referred to as an example5-2. Table 24 lists the composition of the respective layers and thephysical properties of the respective layers of single layer structureincluding the same composition.

                  TABLE 24                                                        ______________________________________                                        RESP MAG LAYER COMP                                                                           θs                                                                              Hcs      Tc                                           (at %)          (degree)                                                                              (kOe)    (°C.)                                 ______________________________________                                        MAG LAYER 51    0.24    3.3     190   TM                                      Pr5 Dy16 Tb3 Fe60 Co15                                                        MAG LAYER 52    0.34    0.5     270   TM                                      Pr24 Tb8 Fe31 Co37                                                            MAG LAYER 53    0.22    13.0    140   TM                                      Pr6 Dy16 Tb6 Fe65 Co10                                                        MAG LAYER 54            0.3     240   TM                                      Pr6 Gd21 Fe66 Co7                                                             MAG LAYER 55            1.2     250   RE                                      Dy30 Fe41 Co29                                                                ______________________________________                                         TM: Transition metal rich                                                     RE: Rare earth element rich                                              

With respect to this example 5-2, the Kerr rotational angle θm in thewavelength of 500 nm is 0.63° when measured from the substrate side. Asthe result of the overwrite and reproduction test under theafore-mentioned specifications, the bit error rate is 1.1×10⁻⁵.Therefore, this example 5-2 is more advantageous than the comparativeexample.

With respect to the second magnetic layer 52 of the embodiment 5 shownin FIG. 9, in general, it is preferable to satisfy the followingrelationship from the standpoint of recording and reproducingcharacteristics:

    10≦x≦35 (at %)

    1≦y≦15 (at %)

where x (at %) denotes the composition ratio of the light rare earthelement, and y (at %) denotes the composition ratio of the heavy rareearth element.

Further, in the example 5-1, other samples in which the composition ofthe fourth magnetic layer 54 was replaced with the compositionNd23Tb8Fe31Co38 (at %) the same as that of the second magnetic layer 52were prepared, and the test the same as above was made. This example isreferred to as an example 5-3, hereinafter. As the result of theoverwrite and reproducing test with respect to the example 5-3, the biterror rate is 2.2×10⁻⁴. This example 5-3 is also more advantageous thanthe comparative example.

This example 5-3 is advantageous in manufacturing cost, because thesputtering target can be used in common for forming the second magneticlayer 52 and the fourth magnetic layer 54 during the process ofmanufacturing the multilayer magnetic film. The reason why the secondmagnetic layer 52 and the fourth magnetic layer 54 can be formed of thesame composition is that there exists the common composition range inwhich the coercivity and Curie temperature suitable for both the secondmagnetic layer 52 and the fourth magnetic layer 54 (control layer) canbe obtained. Here, the common range in which the physical propertiesoptimum both for the second and fourth magnetic layers 52 and 54 are 2.0kOe or less in coercivity and 180° C. or more in Curie temperature Tc.

EMBODIMENT 6 (NdCo)

FIG. 13 is a side cross-sectional view showing this embodiment 6. Afirst protective layer 14 with a thickness of 700 angstrom, a firstmagnetic layer 61 with a thickness of 80 angstrom, a second magneticlayer 62 with a thickness of 80 angstrom, a third magnetic layer 63 witha thickness of 800 angstrom, and a second protective layer 15 with athickness of 800 angstrom are laminated in sequence on a polycarbonatesubstrate 10 by magnetron sputtering.

In the sample (example 6-1) prepared for the evaluation test, the firstmagnetic layer 61 is Nd5.9Dy21.9Fe51.8Co20.4 (at %); the second magneticlayer 62 is Nd21.0Co79.0 (at %); and the third magnetic layer 63 isNd5.9Dy15.9Fe61.8Co10.4 (at %). Further, the protective layers 14 and 15are AlSiN.

The comparative example used for the evaluation test of this embodiment6 is the same as the embodiment 6 in the structure shown in FIG. 13 andin the thicknesses of the respective layers, except the composition ofthe first and third magnetic layers 61 to 63 as follows: the firstmagnetic layer 61 is Tb20.0Fe48.9Co31.1 (at %); the second magneticlayer 62 is Nd21.0Co79.0; and the third magnetic layer 63 isTb20.0Fe4.8Co31.1 (at %). Further, the protective layer is AlSiN.

FIG. 14 shows the Kerr rotational angles of the example 6-1 and thecomparative example when measured from the substrate side, in which thenumeral 601 denotes the example 6-1 and 602 denotes the comparativeexample. This graph indicates clearly that the Kerr rotational angle ofthe example 6-1 is larger than that of the comparative example in theshort wavelength range 400 to 600 nm.

The test results which indicate that the example 6-1 is moreadvantageous than the comparative example in the dynamic characteristicswill be described hereinbelow. The laser beam is a SHG light beam of 532nm generated by a semiconductor exited Nd-YAG laser and obtained througha KTiOPO4 element. The diameter of the laser spot is 0.8 μm. A 45 degreedifferential detection method is adopted, and a PIN photodiode is usedas an optical detector. Information signals from 5.0 to 7.0 MHz arewritten by magnetic field modulation at a linear speed of 5.7 m/sec.FIG. 15 shows the S/N ratio obtained when the signal are reproduced, inwhich the numeral 601 denotes the example 6-1 and 602 denotes thecomparative example. FIG. 15 indicates that the S/N ratio of the example6-1 is higher than that of the comparative example, so that the example6-1 is suitable for the high density signal recording.

Further, in the above embodiment 6, the composition of the firstmagnetic layer 61 is not limited to only the afore-mentioned compositionNdDyFeCo. The same effect can be obtained if the composition is a lightrare earth element--heavy rare earth element--transition metal alloyincluding a light rare earth element (e.g., Nd) and provided with thecharacteristics for forming a perpendicular magnetized film, forinstance such as NdDyTbFeCo, NdTbFeCo, etc. Further, the composition ofthe third magnetic layer 63 is not limited to only the above-mentionedNdDyTbFeCo. The same effect can be obtained when NdDyTbFeCo, TbFeCo,TbFeCoCr, etc. is used as the composition of the third magnetic layer63, as far as the desired recording sensitivity is satisfied.

EMBODIMENT 7 (NdFeCo)

The structure of this embodiment 7 is the same as that shown in FIG. 13.A first protective layer 14 with a thickness of 700 angstrom, a firstmagnetic layer 61 with a thickness of 100 angstrom, a second magneticlayer 62 with a thickness of 80 angstrom, a third magnetic layer 63 witha thickness of 800 angstrom, and a second protective layer 15 with athickness of 800 angstrom are laminated in sequence on a polycarbonatesubstrate 10 by magnetron sputtering.

In the sample (example 7-1) prepared for the evaluation test, the firstmagnetic layer 61 is Nd5.9Dy21.9Fe51.8Co20.4 (at %); the second magneticlayer 62 is Nd19.0Fe40.0Co41.0 (at %); and the third magnetic layer 63is Nd5.9Dy21.9Fe61.8Co10.4 (at %). Further, the protective layers 14 and15 are AlSiN.

Further, as the sample (example 7-2) prepared for the evaluation test,the first magnetic layer 61 is Nd5.9Dy21.9Fe51.8Co20.4 (at %); thesecond magnetic layer 62 is Nd19.0Fe40.0Co41.0 (at %); and the thirdmagnetic layer 63 is Tb21.0Fe74.0Co5.0 (at %). Further, the protectivelayers 14 and 15 are AlSiN.

Further, as the sample (comparative example 7-1 prepared for theevaluation test, the first magnetic layer 61 is Tb20.0Fe48.9Co31.1 (at%); the second magnetic layer 62 is Nd19.0Fe40.0Co41.0 (at %); and thethird magnetic layer 63 is Tb20.0Fe48.9Co31.1 (at %). Further, theprotective layers 14 and 15 are AlSiN.

FIG. 16 shows the Kerr rotational angles of the examples 7-1 and 7-2 andthe comparative example 7-1 when measured from the substrate side, inwhich the numeral 701 denotes the example 7-1, 702 denotes the example7-2 and 703 denotes the comparative example 7-1. This graph indicatesclearly that the Kerr rotational angles of the examples 7-1 and 7-2 arelarger than that of the comparative example 7-1 in the short wavelengthrange 400 to 600 nm.

The test results which indicate that the example 7-1 is moreadvantageous than the comparative example 7-1 in the recording andreproducing characteristics will be described hereinbelow. The laserbeam is a SHG light beam of 532 nm generated by a semiconductor exitedNd-YAG laser and obtained through a KTiOPO4 element. The diameter of thelaser spot is 0.8 μm. A 45 degree differential detection method isadopted, and a PIN photodiode is used as an optical detector.Information signals from 5.0 to 7.0 MHz are written by magnetic fieldmodulation at a linear speed of 5.7 m/sec. FIG. 17 shows the S/N rationobtained when the signal are reproduced, in which the numeral 701denotes the example 7-1 and 703 denotes the comparative example. FIG. 17indicates that the S/N ratio of the example 7-1 is higher than that ofthe comparative example 7-1, so that the example 7-1 is suitable for thehigh density signal recording.

Further, in the above embodiment 7, the composition of the firstmagnetic layer 61 is not limited to only the afore-mentioned compositionNdDyFeCo. The same effect can be obtained if the composition is a lightrare earth element--heavy rare earth element--transition metal alloyincluding a light rare earth element (e.g., Nd) and provided with thecharacteristics for forming a perpendicular magnetized film, forinstance such as NdDyTbFeCo, NdTbFeCo, etc. Further, in this embodiment7, the composition of the third magnetic layer 63 is not limited to onlythe above-mentioned NdDyTbFeCo. The same effect can be obtained whenNdDyTbFeCo, TbFeCo, TbFeCoCr, etc. is used as the composition of thethird magnetic layer 63, as far as the desired recording sensitivity issatisfied.

In this embodiment 7, it is desirable that the content of Co in thefirst magnetic layer 61 is larger than that of the second magnetic layer62. This can be verified by the following test.

In the sample (comparative example 7-2) prepared for the verificationtest, the first magnetic layer 61 is Nd5.9Dy21.9Fe62.2Co10.0 (at %); thesecond magnetic layer 62 is Nd19.0Fe40.0Co41.0 (at %); and the thirdmagnetic layer 63 is Nd5.9Dy21.9Fe61.8Co10.4 (at %). Further, theprotective layers 14 and 15 are AlSiN.

With respect to both the example 7-1 and the comparative example 7-2,the S/N ratio of the signal recorded and reproduced by a laser beam witha wavelength of 532 nm is measured in the same way as described before,and further the optimum value of the recording power sensitivity ismeasured. Here, the optimum value of the recording power sensitivity isdefined as the recording power which can minimize the secondary higherharmonic wave of the reproduced signal under the condition that therecording frequency is fixed at 5.0 MHz (duty: 50%). The linear speed atthe recording operation in this test is 5.7 m/sec. FIG. 18 shows therelationship between the recording frequency and the S/N ratio of themeasured reproduced signal. In FIG. 18, the numeral 701 denotes theexample 7-1, and 703 denotes the comparative example 7-2. This graphindicates clearly that the S/N ratio of the example 7-1 is larger thanthat of the comparative example 7-2. Further, with respect to theoptimum value of the recording power sensitivity, there exists nodifference in the optimum power between the example 7-1 and thecomparative example 7-2 within the precision of 0.1 mW. Therefore, it isunderstood from the above-mentioned facts that the S/N ratio of thereproduced signal can be increased by determining the Co compositionratio of the first magnetic layer 61 more than that of the thirdmagnetic layer 63, without changing the recording power sensitivity.

EMBODIMENT 8 (ULTRAHIGH RESOLUTION)

FIG. 19 is a side cross-sectional view showing the embodiment 8, whichis a magnetic ultrahigh resolution medium. A first protective layer 14with a thickness of 700 angstrom, a first magnetic layer 81 with athickness of 80 angstrom, a second magnetic layer 82 with a thickness of80 angstrom, a third magnetic layer 83 with a thickness of 900 angstrom,a fourth magnetic layer 84 with a thickness of 600 angstrom, and asecond protective layer 15 with a thickness of 800 angstrom arelaminated in sequence on a polycarbonate substrate 10 by magnetronsputtering. The first, second and third magnetic layers 81, 82 and 83constitute a reproducing film 87. The fourth magnetic layer 84 functionsas a recording film.

For the evaluation test, three following samples (referred to asexamples 8-1 to 8-3) were prepared. In the example 8-1, the first andthird magnetic layers 81 and 83 are formed of NdDyFeCo of TM rich, whoseCurie temperature is 130° C. and whose coercivity is 2.3 kOe at roomtemperature. In the example 8-2, the first magnetic layer 81 is formedof NdDyFeCo of TM rich, whose Curie temperature is 130° C. and whosecoercivity is 2.3 kOe at room temperature, and the third magnetic layer83 is formed of TbFeCo of TM rich, whose Curie temperature is 145° C.and whose coercivity is 2.6 kOe at room temperature. In the example 8-3,the first and third magnetic layers 81 and 83 are formed of TbFeCo of TMrich, whose Curie temperature is 145° C. and whose coercivity is 2.6 kOeat room temperature. Further, in all the examples 8-1, 8-2 and 8-3, thesecond magnetic layer 82 is formed of Nd21.0Co79.0 (at %), the fourthmagnetic layer (the recording layer) 84 is formed of NdDyFeCo of RErich, whose Curie temperature is 205° C. and whose coercivity is 11.5kOe at room temperature, and the protective layers 14 and 15 are AlSiN.

Further, FIG. 20 is a side cross-sectional view showing a comparativeexample 8-1 used for the evaluation test. A first protective layer 14with a thickness of 700 angstrom, a reproducing film 83' of single layerstructure with a thickness of 900 angstrom, a fourth magnetic layer(recording layer) 84 with a thickness of 600 angstrom, and a secondprotective layer 15 with a thickness of 800 angstrom are laminated insequence on a polycarbonate substrate 10 by magnetron sputtering. Thereproducing film 83 is formed of TbFeCo of TM rich, whose Curietemperature is 145° C. and whose coercivity is 2.6 kOe at roomtemperature, and the recording layer 84 is formed of TbFeCo of RE rich,whose Curie temperature is 205° C. and whose coercivity is 12.3 kOe atroom temperature. Further, the protective layers 14 and 15 are AlSiN.

FIG. 21 shows the Kerr rotational angles of the examples 8-1 to 8-2 andthe comparative example 8-1 when measured from the substrate side, inwhich the numeral 801 denotes the example 8-1, 802 denotes the example8-2 and 803 denotes the example 8-3, and 804 denotes the comparativeexample 8-1. FIG. 21 indicates clearly that the Kerr rotational anglesof the examples 8-1 to 8-2 are larger than that of the comparativeexample 8-1 in the short wavelength range 400 to 600 nm.

The test results which verify that the examples 8-1 to 8-3 are moreadvantageous than the comparative example 8-1 in the dynamiccharacteristics will be described hereinbelow.

As the laser beam, a SHG light beam of 532 nm generated by asemiconductor exited Nd-YAG laser and obtained through a KTiOPO4 elementis used. The diameter of the laser spot is 0.8 μm. A 45 degreedifferential detection method is adopted, and a PIN photodiode is usedas an optical detector. Information signals from 5.0 to 7.0 MHz arerecorded by magnetic field modulation at a linear speed of 5.7 m/sec.After the signal recording, the recorded domains are left only in therecording film by applying an initial magnetic field of 3.0 kOe. Theapplied initial magnetic field is opposite in direction to the signalrecording magnetic field. The recorded signal is reproduced bytranscribing the recorded domains in the recording layer to thereproducing layer on the basis of a reproducing laser power. FIG. 22shows the S/N ration measured when the signal is reproduced, in whichthe numeral 801 denotes the example 8-1, 802 denotes the example 8-2,803 denotes the example 8-3, and 804 denotes the comparative example8-1. FIG. 22 indicates that the S/N ratios of the examples 8-1 to 8-3are higher than that of the comparative example 8-1, so that theexamples 8-1 to 8-3 are suitable for the high density signal recording.

EMBODIMENT 9 (OVERWRITE)

This embodiment 9 is an overwrite medium formed with an exchange-coupledrecording film and an auxiliary film. FIG. 23 is a side cross-sectionalview showing the embodiment 8. A first protective layer 14 with athickness of 700 angstrom, a first magnetic layer 91 with a thickness of80 angstrom, a second magnetic layer 92 with a thickness of 50 angstrom,a third magnetic layer 93 with a thickness of 800 angstrom, a fourthmagnetic layer 94 with a thickness of 1000 angstrom, and a secondprotective layer 15 with a thickness of 800 angstrom are laminated insequence on a polycarbonate substrate 10 by magnetron sputtering. Thefirst, second and third magnetic layers 91, 92 and 93 constitute arecording film 97. The fourth magnetic layer 84 functions as anauxiliary film.

For the evaluation test, four following samples (referred to as examples9-1 to 9-4) were prepared. In the example 9-1, the first and thirdmagnetic layers 91 and 93 are formed of NdDyFeCo of TM rich, whose Curietemperature is 130° C. and whose coercivity is 11.1 kOe at roomtemperature. In the example 9-2, the first magnetic layer 91 is formedof NdDyFeCo of TM rich, whose Curie temperature is 130° C. and whosecoercivity is 11.1 kOe at room temperature, and the third magnetic layer93 is formed of TbFeCo of TM rich, whose Curie temperature is 135° C.and whose coercivity is 12.5 kOe at room temperature. In the example9-3, the first and third magnetic layers 91 and 93 are formed of TbFeCoof TM rich, whose Curie temperature is 135° C. and whose coercivity is12.5 kOe at room temperature. In the example 9-4, the first magneticlayer 91 is formed of NdDyFeCo of TM rich, whose Curie temperature is190° C. and whose coercivity is 3.5 kOe at room temperature, and thethird magnetic layer 93 is formed of NdDyFeCo of TM rich, whose Curietemperature is 130° C. and whose coercivity is 11.1 kOe at roomtemperature.

Further, in all the examples 9-1, 9-2, 9-3 and 9-4, the second magneticlayer 92 is formed of Nd21.0Co79.0 (at %), the fourth magnetic layer(the auxiliary film) 94 is formed of DyGdFeCo of RE rich, whose Curietemperature is 255° C. and whose coercivity is 1.5 kOe at roomtemperature, and the protective layers 14 and 15 are AlSiN.

Further, a comparative example 9-1 was prepared. FIG. 24 is a sidecross-sectional view showing the comparative example 9-1 used for anevaluation test. A first protective layer 14 with a thickness of 700angstrom, a recording film 93' with a thickness of 900 angstrom, anauxiliary film 94 with a thickness of 1000 angstrom, and a secondprotective layer 15 with a thickness of 800 angstrom are laminated insequence on a polycarbonate substrate 10 by magnetron sputtering. Therecording film 93' is formed of TbFeCo of TM rich, whose Curietemperature is 135° C. and whose coercivity is 12.5 kOe at roomtemperature, and the auxiliary film 94 is formed of DyGdFeCo of RE rich,whose Curie temperature is 255° C. and whose coercivity is 1.5 kOe atroom temperature. Further, the protective layers 14 and 15 are AlSiN.

FIG. 25 shows the Kerr rotational angles of the examples 9-1 to 9-4 andthe comparative example 9-1 when measured from the substrate side, inwhich the numeral 901 denotes the example 9-1, 902 denotes the example9-2 and 903 denotes the example 9-3, and 904 denotes the example 9-4,and 905 denotes the comparative example 9-1. FIG. 25 indicates clearlythat the Kerr rotational angles of the examples 9-1 to 8-4 are largerthan that of the comparative example 9-1 in the short wavelength range400 to 600 nm.

The test results which verify that the examples 9-1 to 9-4 are moreadvantageous than the comparative example 9-1 in the dynamiccharacteristics will be described hereinbelow.

As the laser beam, a SHG light beam of 532 nm generated by asemiconductor exited Nd-YAG laser and obtained through a KTiOPO4 elementis used. The diameter of the laser spot is 0.8 μm. A 45 degreedifferential detection method is adopted, and a PIN photodiode is usedas an optical detector. Information signals are recorded and reproducedat a linear speed of 5.7 m/sec. After a signal of 3.0 MHz had beenpreviously written, another signal of 5.0 to 7.0 MHz is overwritten byan AOM element by optical modulation. Before overwrite, cap-typerecording domains are left only in the auxiliary film 94 by applying aninitial magnetic field of 4.0 koe. The applied initial magnetic field isopposite in direction to the signal recording magnetic field. FIG. 26shows the S/N ratio measured after overwritten, in which the numeral 901denotes the example 9-1, 902 denotes the example 9-2, 903 denotes theexample 9-3, 904 denotes the example 9-4, and 905 denotes thecomparative example 9-1. FIG. 26 indicates that the S/N ratios of theexamples 9-1 to 9-4 are higher than that of the comparative example 9-1,so that the examples 9-1 to 9-4 are suitable for the high density signalrecording.

Further, in the structure shown in FIG. 23, the same effects as abovecan be obtained, even if the compositions of the first and thirdmagnetic layers 91 and 93 are different from the above-mentionedexamples 9-1 to 9-4, as far as the first and third magnetic layers 91and 93 are formed of a rare earth element--transition alloy ofperpendicular magnetized film characteristics and further the thirdmagnetic layer 93 and the auxiliary film 94 can satisfy the overwriteconditions of exchange coupled type.

EMBODIMENT 10 (OVERWRITE)

This embodiment 10 is also of overwrite medium. FIG. 27 is a sidecross-sectional view showing the embodiment 10. A first protective layer14 with a thickness of 700 angstrom, a first magnetic layer 101 with athickness of 80 angstrom, a second magnetic layer 102 with a thicknessof 50 angstrom, a third magnetic layer 103 with a thickness of 600angstrom, a fourth magnetic layer 104 with a thickness of 100 angstrom,a fifth magnetic layer 105 with a thickness of 800 angstrom, and asecond protective layer 15 with a thickness of 800 angstrom arelaminated in sequence on a polycarbonate substrate 10 by magnetronsputtering. The first, second and third magnetic layers 101, 102 and 103constitute a recording film 107. The fourth magnetic layer 104 functionsas an intermediate film. The fifth magnetic layer 105 functions as anauxiliary film.

For the evaluation test, a sample (referred to as an example 10-1) wasprepared. In the example 10-1, the first and third magnetic layers 101and 103 are formed of NdDyFeCo of TM rich, whose Curie temperature is130° C. and whose coercivity is 11.1 kOe at room temperature. The secondmagnetic layer 102 is formed of NdDyFeCo of TM rich, whose Curietemperature is 130° C. and whose coercivity is 11.1 kOe at roomtemperature. The fourth magnetic layer (the intermediate film) 104 isformed of GdFeCo of TM rich, whose Curie temperature is 220° C. andwhose coercivity is 0.3 kOe at room temperature. The fifth magneticlayer (auxiliary layer) 105 is formed of DyGdFeCo of RE rich, whoseCurie temperature is 255° C. and whose coercivity is 1.5 kOe at roomtemperature. Further, the protective layers are AlSiN.

FIG. 28 shows the Kerr rotational angles of the example 10-1 and thecomparative example 9-1 when measured from the substrate side, in whichthe numeral 1001 denotes the example 10-1, and 1002 denotes thecomparative example 9-1. FIG. 28 indicates clearly that the Kerrrotational angle of the example 10-1 is larger than that of thecomparative example 9-1 in the short wavelength range 400 to 600 nm.

The test results which verify that the example 10-1 is more advantageousthan the comparative example 9-1 in the dynamic characteristics will bedescribed hereinbelow. The recording and reproducing test specificationsare the same as those of the embodiment 9. FIG. 29 shows the S/N ratiomeasured when the signal is reproduced. FIG. 29 indicates that the S/Nratio of the example 10-1 is higher than that of the comparative example9-1, so that the example 10-1 is suitable for the high density signalrecording.

EMBODIMENT 11 (OVERWRITE)

This embodiment 11 is also of overwrite medium. The basic structure ofthis embodiment 11 is the same as that of the embodiment 10 shown inFIG. 27. A first protective layer 14 with a thickness of 70 angstrom, afirst magnetic layer 101 with a thickness of 80 angstrom, a secondmagnetic layer 102 with a thickness of 50 angstrom, a third magneticlayer 103 with a thickness of t (selected from values between 400 and1000 angstrom), a fourth magnetic layer 104 with a thickness of 100angstrom, a fifth magnetic layer 105 with a thickness of 800 angstrom,and a second protective layer 15 with a thickness of 800 angstrom arelaminated in sequence on a polycarbonate substrate 10 by magnetronsputtering. The first, second and third magnetic layers 101, 102 and 103constitute a recording film 107. The fourth magnetic layer 104 functionsas an intermediate film. The fifth magnetic layer 105 functions as anauxiliary film.

For the evaluation test, a sample (referred to as an example 11-1) wasprepared. In the example 11-1, the first and third magnetic layers 101and 103 are formed of NdDyFeCo of TM rich, whose Curie temperature is145° C. and whose coercivity is 10.2 kOe at room temperature. The secondmagnetic layer 102 is formed of Nd21.0Co79.0 (at %). The fourth magnetic(intermediate) film 104 is formed GdFeCo of TM rich, whose Curietemperature is 220° C. and whose coercivity is 0.3 kOe at roomtemperature. The auxiliary layer 105 is formed of DyGdFeCo of RE rich,whose Curie temperature is 255° C. and whose coercivity is 1.5 kOe atroom temperature. Further, the protective layers 14 and 15 are AlSiN.

Further, a comparative example 11-1 was prepared. FIG. 23 is a sidecross-sectional view showing the comparative example 11-1 used for theevaluation test. A first protective layer 14 with a thickness of 700angstrom, a first magnetic layer 91 with a thickness of 80 angstrom, asecond magnetic layer 92 with a thickness of 30 angstrom, a thirdmagnetic layer 93 with a thickness of t (selected from values between400 and 1000 angstrom), an auxiliary film 94 with a thickness of 800angstrom, and a protective layer 15 with a thickness of 800 angstrom arelaminated in sequence on a polycarbonate substrate 10 by magnetronsputtering. The first and third magnetic layers 91 and 93 are formed ofNdDyFeCo of TM rich, whose Curie temperature is 145° C. and whosecoercivity is 10.2 kOe at room temperature. The second magnetic layer 92is formed of Nd21.0Co79.0 (at %). The auxiliary film 94 is formed ofDyGdFeCo of RE rich, whose Curie temperature is 255° C. and whosecoercivity is 1.5 kOe at room temperature. Further, the protectivelayers 14 and 15 were AlSiN.

The test results which verify that the example 11-1 is more advantageousthan the comparative example 11-1 in the dynamic characteristics will bedescribed hereinbelow. The recording and reproducing test specificationsare the same as those of the embodiment 9. The signal of 7 MHz isrecorded at the linear speed of 5.7 m/sec and then reproduced after theinitialization. FIG. 30 shows the S/N ratio measured when the signal isreproduced, in which the abscissa indicates the film thickness t, thenumeral 1101 denotes the example 11-1, and 1102 denotes the comparativeexample 11-1. FIG. 30 indicates that the S/N ratio of the example 11-1is higher than that of the comparative example 11-1, so that the example11-1 is suitable for the high density signal recording.

Further, in this embodiment 11, the effect the same as above can beobtained even if the composition of the intermediate film 104 isdifferent from the above, as far as the intermediate 104 is a magneticlayer including Gd and provided with small perpendicular magnetizedanisotropic characteristics. Further, the same effect as above can beobtained if the intermediate layer 104 is formed of NdCo or NdFeCo.

EMBODIMENT 12 (ARTIFICIAL LATTICE)

FIG. 31 is a side cross-sectional view showing the embodiment 12. Afirst protective layer 14 with a thickness of 700 angstrom, a firstrecording film 121 with a thickness of 440 angstrom, a second recordingfilm 122 with a thickness of 800 angstrom, a protective layer 15 with athickness of 800 angstrom are laminated in sequence on a polycarbonatesubstrate 10 by magnetron sputtering. Here, the first recording film 121is formed by laminating first magnetic layers 123 each with a thicknessof 20 angstrom and second magnetic layers 124 each with a thickness of10 angstrom alternately into a multilayer structure (referred to as anartificial lattice). In this first recording film 121, a first magneticlayer 123 is formed at the interface with respect to the firstprotective layer 14 and also at the interface with respect to the secondprotective layer 15. The artificial lattice of the first recording film121 can be formed by the instrument as shown in FIG. 32. In more detail,a sputter chamber is partitioned by a partition plate 128 into two zones131A and 131B. Within the respective zones 131A and 131B, two sputteringtargets 129 and 130 are placed for forming the respective magneticlayers 123 and 124. When the substrate holder 126 is rotated, thesubstrate 125 is passed alternately through the two chamber zones 131Aand 131B. Therefore, the first magnetic layer 123 can be formed withinone zone 131A and the second magnetic layer 124 can be formed within theother zone 131B. The thicknesses of the respective magnetic layers 123and 124 can be controlled by the sputtering time for the respectivezones 131A and 131B. For the evaluation test, three following samples(referred to as examples 12-1 to 12-3) were prepared. In the example12-1, the first magnetic layers 123 is formed of Nd5.9Dy21.9Fe51.8Co20.4(at %), the second magnetic layer 124 is formed of Nd21.0Co79.0 (at %),and the second recording film 122 is formed of Nd5.9Dy15.9Fe61.8Co10.4(at %). In the second example 12-2, the first magnetic layers 123 isformed of Nd5.9Dy21.9Fe51.8Co20.4 (at %), the second magnetic layer 124is formed of Nd21.0Co79.0 (at %), and the second recording film 122 isformed of Tb20.0Fe48.9Co31.1 (at %). In the third example 12-3, thefirst magnetic layers 123 is formed of Tb20.0Fe48.9Co31.1 (at %), thesecond magnetic layer 124 is formed of Nd21.0Co79.0 (at %), and thesecond recording film 122 is formed of Tb20.0Fe48.9Co31.1 (at %).Further, in the respective examples 12-1, 12-2 and 12-3, the protectivelayers 14 and 15 are both formed of AlSiN.

Further, a comparative example 12-1 was prepared. FIG. 20 is a sidecross-sectional view showing the comparative example 12-1. A firstprotective layer 14 with a thickness of 700 angstrom, a first magneticlayer 61 with a thickness of 80 angstrom, a second magnetic layer 62with a thickness of 80 angstrom, a third magnetic layer 63 with athickness of 800 angstrom, and a second protective layer 15 with athickness of 800 angstrom are laminated in sequence on a polycarbonatesubstrate 10 by magnetron sputtering. The first and second magneticlayers 61 and 62 constitute a first recording film, and the thirdmagnetic layer 63 constitutes a second recording film. The firstmagnetic layer 61 is formed of Tb20.0Fe48.9Co31.1 (at %), the secondmagnetic layer 62 is formed of Nd21.0Co79.0, and the third magneticlayer 63 is formed of Tb20.0Fe48.9Co31.1 (at %). Further, the protectivelayers 14 and 15 are formed of AlSiN.

FIG. 33 shows the Kerr rotational angles of the examples 12-1 to 12-3and the comparative example 12-1 obtained when measured from thesubstrate side, in which the numeral 1201 denotes the example 12-1, 1202denotes the example 12-2, and 1203 denotes the example 12-3, and 1204denotes the comparative example 12-1. FIG. 33 indicates clearly that theKerr rotational angles of the examples 12-1 to 12-3 are larger than thatof the comparative example 12-1 in the short wavelength range 400 to 600nm.

The test results which verify that the examples 12-1 to 12-3 are moreadvantageous than the comparative example in the dynamic characteristicswill be described hereinbelow. As the laser beam, a SHG light beam of532 nm generated by a semiconductor exited Nd-YAG laser and obtainedthrough a KTiOPO4 element is used. A PIN photodiode is used as anoptical detector. Information signals from 5.0 to 7.0 MHz are recordedby magnetic field modulation at a linear speed of 5.7 m/sec. FIG. 34shows the S/N ratio measured when the signal is reproduced. FIG. 34indicates that the S/N ratios of the examples 12-1 to 12-3 are higherthan that of the comparative example, so that the examples 12-1 to 12-3are suitable for the high density signal recording.

Further, the second recording film 122 is not limited to only theNdDyFeCo. The same effect can be obtained as far as the second recordingfilm 122 is formed of a rare earth element--transition metal alloy suchas NdDyTbFeCo, TbFeCo, TbFeCoCr, etc. which can satisfy a desiredrecording sensitivity. Further, the same effect can be obtained, even ifthe first magnetic layer 123 is different from the above-mentionedcomposition, as far as the first magnetic layer 123 is formed of a rareearth element--transition metal alloy provided with perpendicularmagnetization characteristics. Further, in the artificial lattice of thefirst recording film 121, the same effect can be obtained even if thethicknesses of the first and second magnetic layers 123 and 124 are notuniform, respectively. Further, the second magnetic layer 124 can beformed of NdFeCo or NdTbFeCo (however, Tb must be less than 1.5 at %).

EMBODIMENT 13 (ARTIFICIAL LATTICE)

FIG. 35 is a side cross-sectional view showing the embodiment 13. Afirst protective layer 14 with a thickness of 700 angstrom, a firstrecording film 131 with a thickness of 1030 angstrom, and a protectivelayer 15 with a thickness of 800 angstrom are laminated in sequence on apolycarbonate substrate 10 by magnetron sputtering. Here, the firstrecording film 131 is formed by laminating first magnetic layers 132each with a thickness of 30 angstrom and second magnetic layers 133 eachwith a thickness of 10 angstrom alternately into a multilayer structure(referred to as artificial lattice). In this first recording film 131,the first magnetic layer 132 is formed at the interface with respect tothe first protective layer 14 and also at the interface with respect tothe second protective layer 15. For the evaluation test, a followingsample (referred to as example 13-3) was prepared. In the example 13-1,the first magnetic layers 132 is formed of Nd5.9Dy21.9Fe51.8Co20.4 (at%), the second magnetic layer 133 is formed of Nd21.0Co79.0 (at %), andthe protective layers 14 and 15 are both formed of AlSiN. The artificiallattice of the recording film 131 is formed by use of the instrument asshown in FIG. 32.

Further, as the comparative example, the afore-mentioned comparativeexample 12-1 is used.

FIG. 36 shows the Kerr rotational angles of the example 13-1 and thecomparative example 12-1 obtained when measured from the substrate side,in which the numeral 1301 denotes the example 13-1, and 1302 denotes thecomparative example 12-1. FIG. 36 indicates clearly that the Kerrrotational angle of the example 13-1 is larger than that of thecomparative example 12-1 in the short wavelength range 400 to 600 nm.

The test results which verify that the example 13-1 is more advantageousthan the comparative example 12-1 in the dynamic characteristics will bedescribed hereinbelow. The recording and reproducing test was made inaccordance with the specifications basically the same as that used forthe afore-mentioned embodiment 11. That is, information signals from 5.0to 7.0 MHz were recorded by magnetic field modulation at a linear speedof 5.7 m/sec. FIG. 37 shows the S/N ratio measured when the signals arereproduced. FIG. 37 indicates that the S/N ratio of the example 13-1 ishigher than that of the comparative example 12-1, so that the example13-1 is suitable for the high density signal recording.

Further, the first magnetic layer 132 is not limited only to theabove-mentioned NdDyFeCo. The same effect can be obtained, as far as thefirst magnetic layer 132 is formed of a rare earth element--transitionmetal alloy such as NdDyTbFeCo, TbFeCo, TbFeCoCr, etc. which can satisfya desired recording sensitivity. Further, in the artificial lattice ofthe recording film 131, the same effect can be obtained even if thethicknesses of the first and second magnetic layers 132 and 133 are notuniform, respectively.

EMBODIMENT 14 (REFLECTION STRUCTURE+ARTIFICIAL LATTICE)

FIG. 38 is a side cross-sectional view showing the embodiment 14. Afirst protective layer 14 with a thickness of 700 angstrom, a recordingfilm 141 with a thickness of 440 angstrom, a protective layer 15 with athickness of 250 angstrom, and a reflection layer 142 with a thicknessof 800 angstrom are laminated in sequence on a polycarbonate substrate10 by magnetron sputtering. Here, the recording film 141 is formed bylaminating first magnetic layers 143 each with a thickness of 30angstrom and second magnetic layers 144 each with a thickness of 10angstrom alternately into a multilayer structure so as to form theartificial lattice. In the recording film 141, a first magnetic layer143 is formed at the interface with respect to the protective layers 14and 15, respectively.

For the evaluation test, a following sample (referred to as example14-1) was prepared. In the example 14-1, the first magnetic layers 143is formed of Nd5.9Dy21.9Fe51.8Co20.4 (at %), the second magnetic layer144 is formed of Nd21.0Co79.0 (at %), the protective layers 14 and 15are formed of AlSiN, and the reflection layer 142 is formed of Al. Theartificial lattice of the recording film 141 is formed by use of theinstrument as shown in FIG. 32.

Further, as the comparative example, the aforementioned comparativeexample 12-1 is used.

FIG. 39 shows the Kerr rotational angles of the example 14-1 and thecomparative example 12-1 obtained when measured from the substrate side,in which the numeral 1401 denotes the example 14-1, and 1402 denotes thecomparative example 12-1. FIG. 39 indicates clearly that the Kerrrotational angle of the example 14-1 is larger than that of thecomparative example 12-1 in the short wavelength range 400 to 600 nm.

The test results which verify that the example 14-1 is more advantageousthan the comparative example 12-1 in the dynamic characteristics will bedescribed hereinbelow. The specifications for the recording andreproducing test are basically the same as those of the embodiment 9.Signals from 5.0 to 7.0 MHz are written by magnetic field modulation ata linear speed of 5.7 m/sec. FIG. 40 shows the S/N ratio measured whenthe signals are reproduced. FIG. 40 indicates that the S/N ratio of theexample 14-1 is higher than that of the comparative example 12-1, sothat the example 14-1 is suitable for the high density signal recording.

Further, the first magnetic layer 143 is not limited to only theNdDyFeCo. The same effect can be obtained as far as the first magneticlayer 143 is formed of a rare earth element--transition metal alloy suchas NdDyTbFeCo, TbFeCo, TbFeCoCr, etc. which can satisfy a desiredrecording sensitivity. Further, in the artificial lattice of therecording film 141, the same effect can be obtained even if thethicknesses of the first and second magnetic layers 143 and 144 are notuniform, respectively. Further, the same effect can be obtained in thecase where the reflection layer 16 is formed of AlTi, AlCr, AlTa, Ag,Cu, Au, Pt or Co. Further, the second magnetic layer 144 can be formedof NdFeCo or NdTbFeCo (however, Tb must be less than 1.5 at %).

EMBODIMENT 15 (4-LAYER SANDWICH+REFLECTION STRUCTURE)

FIG. 41 is a side cross-sectional view showing the embodiment 15. Afirst protective layer 14 with a thickness of 600 angstrom, a firstmagnetic layer 11 with a thickness of 30 angstrom, a second magneticlayer 12 with a thickness of 30 angstrom, a third magnetic layer 13 witha thickness of 50 angstrom, and a fourth magnetic layer 17 with athickness of 90 angstrom, a second protective layer 15 with a thicknessof 200 angstrom, and a reflection layer 16 with a thickness of 600angstrom are laminated in sequence on a polycarbonate substrate 10 bymagnetron sputtering. The first to fourth magnetic layers 11, 12, 13 and17 constitute a recording film 118.

For the evaluation test, a sample (referred to as example 15-1) wasprepared. In the example 15-1, the protective layers 14 and 15 areformed of AlSiN, the reflection layer 16 is formed of Al, the first andthird magnetic layers 11 and 13 are formed of Nd25Tb7Fe33Co35 (at %),and the second and fourth magnetic layers 12 and 17 are formed ofNd6Dy22Fe52Co20 (at %). In the first and third magnetic layers 11 and 13of single layer structure, the Kerr rotational angle is 0.25° inwavelength 500 nm and the coercivity is 10.05 kOe. In the second andfourth magnetic layers 12 and 17 of single layer structure, the Kerrrotational angle is 0.34° in wavelength 500 nm and the coercivity is 0.8kOe.

Further, a comparative example 15-1 was prepared. FIG. 2 is a sidecross-sectional view showing a comparative example used for theevaluation test. A first protective layer 14 of 600 angstrom inthickness, a first magnetic layer 11 of 60 angstrom in thickness, arecording film 12' of single layer structure with a thickness of 200angstrom, a second protective layer 15 of 200 angstrom in thickness, anda reflection layer 16 of 600 angstrom in thickness are laminated on apolycarbonate substrate 10 in sequence by magnetron sputtering. Thematerial of the protective layers 14 and 15 is AlSiN, that of thereflection layer 16 is Al, and that of the recording film 12' isNd6Dy22Fe52Co20 (at %) in the same way as in the second magnetic layer12 of the example 15-1.

The Kerr rotational angle θm of the example 15-1 is 1.05° in wavelengthof 500 nm when measured from the substrate side, and that of thecomparative example 15-1 is 0.60° under the same condition. Further, thecoercivity Hcm of the example 15-1 is 4.8 kOe, and that of thecomparative example 15-1 is 11.8 kOe. In other words, the example 15-1has a larger Kerr rotational angle than that of the comparative example15-1, and additionally a sufficient coercivity Hcm beyond the minimumcoercivity of 2 kOe required in practice. Therefore, the example 15-1can provide magneto-optical recording medium high in S/N ration in theshort wavelength and stable in recording magnetic domains.

Further, another sample (referred to as example 15-2) was prepared. Inthis example 15-2, the composition of the fourth magnetic layer 17 isTb22Fe73Co5 (at %), and the compositions of the other layers are thesame as those of the example 15-1. The Kerr rotational angle of thefourth magnetic layer 17 (Tb22Fe73Co5 (at %)) of single layer structureis 0.18° in wavelength 500 nm and the coercivity thereof is 12.3 kOe.

The Kerr rotational angle θm of the example 15-2 is 0.78° in wavelengthof 500 nm when measured from the substrate side, and the coercivity Hcmof the example 15-2 is 8.9 kOe. As described above, when the fourthmagnetic layer 17 is formed of a composition such as TbFeCo high incoercivity, it is possible to increase the coercivity, while keeping thelarge Kerr rotational angle in the short wavelength range.

Under the consideration of the optimum value of the composition rangesexamined in the embodiment 1, it is preferable that the first and thirdmagnetic layers 11 and 13 satisfy the following relationship from thestandpoint of recording and reproducing characteristics:

    10≦x≦35 (at %)

    1≦y≦15 (at %)

where x (at %) denotes the composition of light rare earth element, andy (at %) denotes the composition of heavy rare earth element.

The respective layer thickness ratios of the magnetic layers 11, 13 and17 must be selected so that both the enhancement of the Kerr rotationalangle and the coercivity of the whole recording film 118 become anappropriate value, respectively.

The film thickness of the recording film 118 is preferably 400 angstromor less. This is because when the thickness of the recording film 118 isthin to this extent, light can transmit therethrough sufficiently, sothat it is possible to make the most of the advantage of adoption of thereflection structure. The inventors prepared the following samples: Afirst protective layer 14 with a thickness of 600 angstrom, a firstmagnetic layer 11 with a thickness of 30 angstrom, a second magneticlayer 12 with a thickness of 30 angstrom, a third magnetic layer 13 witha thickness of 50 angstrom, a fourth magnetic layer 17 with a thicknessx angstrom, a second protective layer 15 with a thickness of 250angstrom, and a reflection layer 16 with a thickness of 1000 angstromare laminated in sequence on a polycarbonate substrate with a thicknessof 1.2 nm. The layer thickness x of the fourth magnetic layer 17 is 90,190, 240, 270 and 320 angstrom, respectively. With respect to theabove-mentioned samples, the respective Kerr rotational angles θm aremeasured from the substrate side. The compositions of the respectivelayers of the samples are the same as those of the example 15-1. Thefilm thickness of the recording films 118 of these samples are 200, 300,350, 380 and 430 angstrom, respectively. The Kerr rotational angles θmin wavelength 500 nm measured from the substrate side are 1.11°, 1.03°,0.80°, 0.77° and 0.62°, respectively. Further, the coercivities are 4.2,4.7, 5.8, 6.2 and 6.6 kOe, respectively. The test results indicate thatthe Kerr rotational angle θm increases with decreasing film thickness ofthe recording film 118.

The same effect as described above can be obtained even if thecompositions of the first and third magnetic layers 11 and 13 are notnecessarily the same with respect to each other. Further, it is possibleto obtain the same effect as described above, if the compositions of thesecond and fourth magnetic layers 12 and 14 are not necessarily the samewith respect to each other.

Further, the Curie temperatures of the first and third magnetic layers11 and 13 are 200° C. or higher.

Without being limited to only the above-mentioned embodiments, thepresent invention can be modified into various embodiments andcompositions. For example, the compositions including Sm as a light rareearth element or the compositions including Gd as a heavy rare earthelement can be adopted as the composition of the magnetic layers. Inparticular, in the case of the light rare earth major range composition,it is possible to adopt the composition including Sm (e.g., NdSmTbFeCo,PrSmTbFeCo, NdPrSmTbFeCo) or these compositions in which Tb is replacedwith DyGd. Further, in both the light and heavy rare earth majorcompositions, it is possible to adopt the compositions including Gd(e.g., NdTbGdFeCo, NdDyGdFeCo, NdGdFeCo, PrTbGdFeCo, or PrGdFeCo, etc.).When Gd is added, although the coercivity is decreased, the Kerrrotational angle increases.

We claim:
 1. A magneto-optical recording medium having as laminations inthe following order at least a protective layer, a first magnetic layer,a second magnetic layer, a third magnetic layer, another protectivelayer, a reflection layer and a transparent substrate, wherein:the firstand third magnetic layers are formed of a rare earth element--transitionmetal alloy provided with characteristics for forming a perpendicularmagnetized film, respectively; the second magnetic layer is formed of alight rare earth element--heavy rare earth element--transition metalalloy; and the first, second and third magnetic layers satisfy thefollowing conditions with respect to Kerr rotational angle in awavelength range of light used for signal reproduction and coercivity,both at room temperature:

    θ1<θ2

    θ3<θ2

    Hc1>Hc2

    Hc3>Hc2

where θ donates the Kerr rotational angle, Hc denotes the coercivity,and suffixal numerals 1, 2 and 3 denote the first, second and thirdmagnetic layers, respectively.
 2. The magneto-optical recording mediumof claim 1, wherein the light rare earth element of the second magneticlayer includes Nd and/or Pr.
 3. The magneto-optical recording medium ofclaim 1, wherein the second magnetic layer satisfies the followingconditions with respect to composition ratio:

    10≦x≦35 (at %)

    1≦y≦15 (at %)

where x denotes a composition ratio of the light rare earth element, andy denotes a composition ratio of the heavy rare earth element.
 4. Themagneto-optical recording medium of claim 1, wherein the rare earthelement of the first and third magnetic layers includes Nd and/or Pr. 5.The magneto-optical recording medium of claim 1, wherein the first andthird magnetic layers are prominent in sublattice magnetization oftransition metal at room temperature, respectively, and further satisfythe following conditions with respect to the Kerr rotational angle andthe coercivity

    θ1>θ3

    Hc1<Hc3.


6. The magneto-optical recording medium of claim 1, wherein the firstmagnetic layer is prominent in sublattice magnetization of transitionmetal at room temperature, the third magnetic layer is prominent insublattice magnetization of rare earth element at room temperature, andfurther the first and third magnetic layers satisfy the followingconditions with respect to the Kerr rotational angle and the coercivity

    θ1>θ3

    Hc1<Hc3.


7. 7. A magneto-optical recording medium comprising:as laminations inthe following order at least a protective layer, a recording film, anauxiliary film of a rare earth element--transition metal alloy, aprotective layer and a transparent substrate, the recording film and theauxiliary film being exchange coupled with respect to each other;wherein the recording and auxiliary films satisfy the followingconditions with respect to coercivity at room temperature and Curietemperature for enabling direct overwrite by optical modulation:

    Hc1>Hc2

    Tc1<Tc2

where Hc denotes the coercivity, Tc denotes the Curie temperature, andsuffixal numerals 1 and 2 denote the recording and auxiliary films,respectively; the recording film is laminations in the following order afirst magnetic layer of a light rare earth element--heavy rare earthelement--transition metal alloy, a second magnetic layer of a light rareearth element--heavy rare earth element--transition metal alloy, and athird magnetic layer of a rare earth element--transition metal alloy onthe substrate; and the first, second and third magnetic layers satisfythe following conditions with respect to Kerr rotational angle andcoercivity, both at room temperature:

    θa<θb

    θc<θb

    Hca>Hcb

    Hcc>Hcb

where θ denotes the Kerr rotational angle, Hc denotes the coercivity,and suffixal letters a, b and c denote the first, second and thirdmagnetic layers, respectively.
 8. The magneto-optical recording mediumof claim 7, wherein the light rare earth element of the second magneticlayer includes Nd and/or Pr.
 9. The magneto-optical recording medium ofclaim 7, wherein the light rare earth element of the first and thirdmagnetic layers includes Nd and/or Pr.
 10. The magneto-optical recordingmedium of claim 7, wherein the second magnetic layer satisfies thefollowing conditions with respect to composition ratio:

    10≦x≦35 (at %)

    1≦y≦15 (at %)

where x denotes a composition ratio of the light rare earth element, andy denotes a composition ratio of the heavy rare earth element.